GETTING INTO the Orvis Fishing School is like being accepted at Harvard. Both institutions are old, prestigious, with quality instruction and a long waiting list. One difference is that Harvard demands proven ability. Not Orvis. So once again I was going to school.

Tuition at Orvis, like Harvard, isn't cheap ($190 for three days of instruction, room and board), which prompted two question:

Just how much is there to learn about fly fishing?

Can it be learned in three days?

If the answer to the second question is yes it would seem the answer to the first is not much. Wrong on both accounts.

A friend and I took the two-hour scenic route from Washington to Boiling Springs, Pa., passing through tidy towns where Main Streets still flourish and stop lights are sparse. When we reached Boiling Springs we headed for the Orvis School at the Allenberry Inn on the Yellow Breeches.

With buildings dating back to 1785 and shadowy sycamores dating even earlier, this country inn alone was worth the drive. There is a comity among boarders that results in strange behaviour, like speaking in passing.

We sat down with the other 30 students for lunch and introductions. Ages ranged from a 10-year-old lad to a 73-year-old widow. At our table there was a retired headmaster and a retired teacher, a newspaper bureau chief, an executive from Campbell Soup and his wife, a lawyer. All were here for the first time; all had some fishing experience.

Tony Skilton, the 30-year-old director of the school, met us after lunch and began a slide-lecture on the art of fishing. A familiar object flashed on the screen.

"Not to insult your intelligence," said Skilton, "but this is a hook."

Skilton's hook lecture was indicative of the thoroughness of the school. When he asked someone to name the four parts of the hook no hands went up.

Skilton went on to describe each piece of equipment the fly fisherman uses and why. Then he talked about trout: the browns, the brooks and the rainbows; then about the insects trout eat.

About midafternoon we switched with the other half of the class and went out on a field to practice casting with Ed Koch, Norm Shires and Ed Shenk. Before long, everyone from the 10-year-old to the 73-year-old had a nice lariat going.

That night Koch lectured on the life cycle of the mayfly and on the importance of choosing the right fly for the right time of the year. He discussed the significance of scouting a stream, observing not only fish habits but insects', too. A session in knot-tying ended the evening.

The first day was over and we hadn't even seen water.

We did the next morning, and it was easy to understand why the Yellow Breeches ranks high in fly-fishing folklore. The banks are wide and green, enclosing a floating insect buffet for a crowd that eats when it chooses - and eats well. In this particular "fish for fun" area - about 1.2 miles of the 40 miles of the limestone Yellow Breeches - nothing under 20 inches is a keeper.

It was a warm, April day in Pennsylvania and other anglers already had waded out in the streams and were casting away. We students were gathered on the bank listening to our instructors explain wet- and dry-fly techniques. Within 10 minutes each of the four instructors had pulled in a trout. Someone joked that those four fish were on the Orvis payroll.

That slice of piscine humor placated me through a morning of lost flies and hooked trees. The fish were there - in fact, they were quite visible - but as Skilton had warned us they are lazy and smart. A check at noontime confirmed we were still students - not one fish landed.

After lunch, two of us slipped off to the fish hatchery a few hundred yards down the road. Owned and operated by the Yellow Breeches Anglers and Conservation Association, the 40,000 trout in the hatchery will enter 9 1/2 miles of stream in the Boiling Springs area periodically through the end of August, assuring a well-stocked stream year 'round. Supported by its 1,000 members, the YBACA is a major reason why the Yellow Breeches remains a top fly fishing spot.

Back at school, Skilton reemphasized the need to keep a close eye on the line and set the hook quickly. He suggested using a nymph fly and looking carefully for shadowy eddies where the trout sit and feed.

Wading out to midstream with the clear, cold water gurgling smoothly by, I watched anglers gracefully lariating up and down the banks. It seemed like a silent movie, only the occasional whirr of the reel or the singing of the birds breaking the symphony of silence. Now and then a couple of malard ducks, breaking through the green of overhanging trees like planes dropping from clouds, came gliding in for perfect landings.

As the sun set and the underside of leaves began to glow, the trout began to rise. We switched to dry flies. Within a few minutes my partner had hooked a small brookie. I was working on the technique of "rolling" the line when a fish came by. My reaction was slow and it had already spit the fly before I could pull the line taut.

Again thatt evening we were in the lecture room where we saw a remarkable nature film, "The Way of A Trout." The film traced the life of a rainbow from birth to full maturity.

The final day, Sunday, was given completely to fishing. Even the instructors were up early on this warm April morning in hopes of catching a good breakfast. March's "Outdoor Life" lists some [WORD ILLEGIBLE] fishing schools across the country and magazines like "Fly Fisherman" carry ads for schools in every issue. The Orvis School, the oldest in the country, has summer classes in Manchester, Vt., (April-August: $180 and booked up rather quickly).

That is happening, Skilton said, because fly fishing is coming into its own. Over the eight years he has been instructing, Skilton has noticed that the age of fishermen is dropping and their dedication is rising. Orvis' sales of fly-tying equipment, a hobby reserved for serious anglers, has risen almost 200 per cent in the last year, he said.