"I know who you are," whispered the principal's secretary as she thrust a manila folder full of instructions for substitute teachers at me and waved me down the dim school hallway.

I was spending the day at Yorktown High, a 20-year-old, beige brick building in an affluent North Arlington neighborhood. As far as I knew only the principal -- and apparently his secretary -- knew my mission: to spend a day as a substitute. I wanted a firsthand look at the world of those retired teachers, graduate students, housewives and retired military who try to spend eight hours in school without a major incident. And all for $30 a day.

I unlocked my dark, empty classroom and opened the folder.

"You should find all these classes and virtually all students cooperative," read an upbeat note from the full-time teacher. "Certainly they should cause absolutely no difficulty. I hope you enjoy the day."

Enjoy the day? When an initial tour of the classroom found an 8-by-10-inch glossy photo of the man I was replacing prominently displayed with a caption labeling him "Public Enemy Number 1"?

No difficulty? When a mock campaign poster in the back of the classroom proclaimed: "Fight for DoNothingism"?

Cooperative? When I would try to start a class discussion about Congress a student would shout, "We want to talk about birth control!"? a

I knew most students greet substitutes with the same delight as snow on a school day -- an excuse for the educational process to crash to a halt. County school administrators had invited me to share that classroom experience, agreeing that I would work without pay and my identity would be disclosed only if someone asked.

No one did.

My first-period class was so relieved to discover a substitute that I was given a standing ovation. In another class exuberant students danced the tango.

The words of one substitute I had interviewed haunted me as the first class filed in: "Students will do to a substitute what they would never do to a regular teacher," she warned.

After the applause they stared at me.

I was ready for a variety of problems. I was on guard for the infamous "close adjusters" -- students who turn the clocks ahead when the substitute isn't looking. I was wise to the students who would dribble water on my chair or pull the old name-switching scam -- where students take each other's names and titter every time they are called by their new alias.

I knew about the substitute at another school who had been locked in a closet for several weeks before. And I had talked personally to a sub who had asked a student on his way to the lavatory to bring her a cup of water and was given a cup of yellowish liquid. Much to her chagrin, the student later drank the liquid to prove it was water.

I was prepared for the more exotic pranks, including students who would pretend to be deaf during roll call. After all, I am an alumna of Hamilton High West in Trenton, N.J. -- where students dissected frogs with switchblades and refined the art teacher intimidation. Students there boasted that the local teachers college refused to send student teachers because too many novices had left the education field after a day in Hamilton High.

Surely the offspring of bureaucrats would behaved than the children of steelworkers. I was to be a "government teacher," instructing five classes of 14 to 28 high school seniors on Congress and political parties.

I was eager to share the knowledge of government I had gained after five years in the nation's capital. Eager that is, until I read my lesson plans. The plans, prepared by the teacher I would replace, called only for the students to have a "silent" review in preparation for a test on the Congress later in the week. "The students should work individually and quietly at their desks," the note from the teacher instructed.

In other words, I was to baby-sit.

I soon found that a substitute is no substitute at all for a "regular" teacher, just a human noise meter, constantly measuring the decibel level and making snap decisions about how much whispering is tolerable.

By the end of the first 52-minute period, I had granted five dubious requests to go to the bathroom. I also had retrieved a wooden lectern from two students with a pocket screwdriver who were booby-trapping it to fall apart when their regular teacher leaned on it.

"Every party needs a pooper . . .," one sang back at me as I attempted to right the lectern.

I had already wearied of telling a room full of seniors to be quiet.

"Don't worry," said one smiling student on his way out the door. "By this afternoon you won't have anybody in class. They'll just cut out when they hear there's s substitute."

He was right. By fifth period my absentee list had grown to seven and was climbing.

The minutes and hours dragged by -- you can only flip through a high school government text so many times. By second period I had developed my own technique for noise control: patrolling the aisles. I learned that even the most arrogant students became silent when they are within six inches of a teacher.

That was at least my feeling until there was an outburst of giggling. Overwhelmed with the fear that my slip was showing, I retreated to my desk.

Third period was listed as my schedule as "p&c." I waited but "p&c" showed, so I finally figured it was my "planning and counseling" period. No student appeared. I read a paperback, "Conjectures of a Guilty Bystander." s

Who can face a tuna fish sandwich before noon?

I ventured down to the smoke-filed teachers lounge where teachers sat silently reading, smoking, doing class work or eating their lunches. I had worried about answering the questions of curious teachers without lying about being a reporter. Nothing to fear -- no one spoke to me.

Back in fifth-period class, the students seemed unduly subdued.

Three students fell asleep right after lunch, one before I had a chance to call the roll. Several students began yawning and putting their heads on their desks.

I wondered if they were hypoglycemic and overdosing on sugar. Then I noticed several sets of red-rimmed eyes.

I began to panic at the thought of 25 sleeping students.

One student walked up to talk to me and another boy in the rear of the class cautioned, "Watch out, he's on acid."

Exasperated, I tried to instigate a discussion. How about comparing the last Congress with the newly elected one? No way. Why don't we discuss the latest developments in the hostage crisis? Nope. What would the students like to discuss?

"We want to talk about birth control," one student shouted. "Yeah, birth control," chimed in a roomful of students at the peak of puberty.

Back to the study hall format.

In my next class I noticed that two boys had slipped into a small conference room adjoining the classroom. What were they doing?

"We're troublemakers," one of them replied. "We decided to isolate ourselves."

The final bell rang and one hulking student whom I quickly calculated to outweigh me by 100 pounds stayed behind. The door was closed, the classroom silent. I looked for telltale signs of a weapon and remembered movies about teachers being brutalized by students. I glanced nervously at the two tiny conference rooms adjacent to the class.

"How have high schools changed since you went to school," he asked shyly.

I heaved a sigh of relief.

Area school officials concede there are problems in the substitute field and say they are working to make improvements. In Fairfax County, the school administration established a substitute committee several years ago and is planning to make some changes in the future.

One shortcoming is that no one seems to represent the interests of the substitute teacher. A representative of the Fairfax Education Association, the largest teachers organization in Northern Virginia, says the group has failed in its effort to win benefits for substitutes, but attempts nonetheless to sign them as members.

". . . We encourage them to join primarily for the liability insurance," says the FEA's Barby Halstead. That can be very handy, some teachers told me, for a substitute who flies off the handle and makes the mistake of whacking an unruly child.

"At some point in time we are going to do a better job of evaluating substitutes and possibly increase their pay," says Larry Byers, personnel coordinator for Fairfax schools. "I think substituting should be more than babysitting . . ."

There's no question that substitutes are becoming a more important factor in public education in the Washington area. School administrators say they can't find enough substitutes and often will accept someone with only two years of college to take the place of a teacher with a bachelor's degree.

The schools say an increasing number of teachers are taking time off to attend school conventions and meetings or even to tend to personal matters, leaving their classrooms in the hands of people like me.

In Arlington and Alexandria, teachers have only to leave their name on a telephone tape recording to get the day off -- a method that some teachers say encourages absenteeism by allowing them to skip school without having to talk to anyone they know.

Maria Gerber substitutes daily in Arlington and claims she never has an out-of-control day.

"I don't find the kids very difficult at all," says the former chemist who gave up her career to rear three children and returned to work -- as a substitute teacher -- 13 years ago. "The substiture's duty is to see that the students don't miss a day of education."

To be effective, Gerber says substitutes need to work at just one or two schools where they can become familiar with the students and well-known as a substitute. She also believes the substitute should try to be prepared to teach.

Gerber is a certified teacher who is fluent in five languages, qualified to teach science, mathematics, English, art, home economics and auto mechanics. The county has offered her several full-time teaching positions. She turned them down.

"Boring, boring, boring," Gerber says of regular teaching. "I wouldn't want to teach the same subject and see the same students every day."

What keeps Gerber coming back? She says it's the enjoyment she derives from working with students. "I think it's much better to associate with young people," she says, "than your contemporaties who are getting menopause pains."