Last summer Barbara Colbert's teen-aged twins, Aaron and Adriene, had enough reduced-fare Metrobus tokens, which are issued to get city children to and from school cheaply, to carry them through the summer -- saving 50 cents on each bus ride they took.

So Adriene's daily trip to work, Aaron's crosstown ride to help an invalid relative, social visits, and other Metrobus travel from their Northeast home cost a dime a ride, instead of the regulation 60 cents.

But starting June 26 the Colberts and thousands of other District residents enjoying cut-rate summer rides will lose their inexpensive mobility, when the D.C. Department of Transportation cracks down on the unauthorized use of school Metrobus tokens.

Transportation officials decided last week to eliminate use of the metal school tokens during off-school periods and replace them with tightly controlled bus tickets issued only to those in summer school programs. Officials said the crackdown would save the city, which pays Metro the 50-cent difference on each ride, an estimated $500,000 during the summer and about $250,000 a year during school holidays and weekends.

But the move also will bring to an end a thriving Washington underground in which the 10-cent tokens are diverted from students and marketed to nonstudents and adults for about a quarter. Moreover, young people who used the tokens for work and play will now pay more or travel less, since an estimated 10,000 school tokens a day were used during the summer vacation.

Transportation and school officials, parents, children, bus drivers and Metro officials tell of tokens being sold in pool halls and from ice cream trucks and hawked on the streets. They have been stolen, bilked from small children, and obtained with pirated and Xeroxed copies of the necessary purchase forms.

That many of the 50,000 to 60,000 student tokens dropped into fareboxes daily during the school year do not fall from the hands of school children is hardly a secret.

"They get on with mud on their boots, hammers on their hips and saws in their hands, dropping school tokens," scoffed one of several bus drivers at the Metro garage at 14th and Decatur Streets NW, all of whom asked not to be identified.

"I worked the Park Road route," added another, "and everybody from six to 60 was using them."

"They're as old as I am," agreed a third, "and I'm 43."

"There's a float out there that can't be contained," said John A. Drayson, assistant director of mass transportation and a principal designer of the new summer system. "We try to stay one step ahead, but we've got 100,000 kids out there working on it full time, and there are only two or three of us here, working against it part time."

Under D.C. law, use of school tokens is limited to students 18 and younger who are enrolled in public or private schools in the District, for travel to and from school or related educational activities. They are sold for 10 cents each in packs of 10 or 20 at more than 100 banks, credit unions, Metro ticket offices and other outlets around town, to anyone presenting an application form signed by a school principal.

There are, however, many ways to obtain the purchase forms and tokens.

About a year ago, for example, transportation officials learned that some banks and savings and loans, which supply tokens to the public as a free service, were selling them without demanding the proper authorizing form.

"I don't think the banks or their employes were doing this intentionally," Drayson said.

To control summer purchases of the tokens last year, the white purchase form was replaced with a pink form.

"The kids were one jump ahead of us," Drayson said. "They simply bought an excess of tokens and hoarded them."

And by the time the pink forms were in use, so was a new scheme -- people were making copying-machine reproductions of the form on pink paper. Drayson said the yellow forms used this summer will be printed on check paper, a stock more difficult to counterfeit.

The many underground outlets for the cheap tokens are common knowledge to school children. A group of boys gathered outside Kelly Miller Junior High, 49th and Brooks Streets NE, last week, for example, swapped tales they had heard of how people misuse the tokens.

"They sell 'em in the candy truck, the ice cream truck, right over here in Deanwood," one boy said.

"You can buy 'em at the pool hall for 20 cents, and people walk around selling them in the street," said another.

At Sousa Junior High, 37th Street and Ely Place SE, principal Ralph H. Neal expressed doubt that his students abuse the tokens.

"We monitor them through issuance of the applications, and we only issue them once a week to students who ride the bus," Neal explained.

Students can, however, buy 20 tokens a week -- twice the number a student would need for a week of roundtrips to school.

In his 10th floor shop at the D.C. school board headquarters, mailroom supervisor Arthur N. Clark said he has "the tightest control I can keep on these things" -- the application slips that are shipped to him by the carton from Metro and store in a padlocked, plywood cabinet until school principals call for them. "But I furnish them [20 books of 100 forms each at a time] to any school that calls, the next day after they call."

"We rely on the schools -- public and private -- to monitor it," Drayson says. "Unfortunately, we are not able to monitor the activities of the schools to be sure they are administering it within the regulations. Some schools are very diligent; others are more casual. It is possible to circumvent the system by coming back twice in a week."

Given the ease with which tokens are secured, bus drivers have the final authority to deny a questionable reduced-fare ride -- something few seem to do. a

"We teach the bus operator to make every effort to collect the proper fare, but it's not always possible," said Donald Fluharty, superintendent of bus operator training. "He uses a lot of his own judgment, to avoid confrontation when he's out there on the route."

"If the person who is trying to abuse is six feet, 220 pounds, the driver is not going to challenge him," Drayson said. "They (the drivers) suffer enough abuse as it is."

Barbara Colbert said she "saved up" tokens by acquiring authorized forms to buy more than were needed during the school year and by frequently driving the twins to St. Anthony's High School. Her husband, an assistant librarian at the Smithsonian Institution, sometimes bought three packs of tokens a week to amass summer tokens, she said.

"Just before school closed I was able to buy up a lot of them, and the bus drivers are so lenient," Colbert said.

One reason the tokens have not been more closely regulated, Drayson said, is because some city officials believe there should be a special low fare for youths.

"There is a reluctance," he said, "to make petty criminals out of every school child. If an adult is abusing the system that's another matter."

City Council member Betty Ann Kane, who said "there should be a youth fare, like the senior citizen fare," said she is all for cutting down on abuses. But she also said she fears that the tight rules may deny students reduced fare rides to museums, libraries and other educational destinations.

The plan to crack down on the misuse of school tokens has angered some people.

"It's another burden for some of these welfare mothers. Somebody's going to have to take up the slack and 10 cents more is not going to balance the budget," said Kimi Gray, president of the resident's council at Kenilworth Courts public housing project. "It's going to cause a lot of hitchhiking."

Nevertheless, the new procedures will be implemented, transit officials said. Summer school students or those in approved programs will fill out forms specifying the exact number of reduced-fare trips necessary. Those forms will be returned to Drayson's office and checked against a list of students supplied by the schools before new yellow purchase forms will be issued -- one for every 10 tickets. Only Metro ticket offices will sell them.

The tighter guidelines may even save money for some who use the tokens. Drayson recalled one angry Northeast mother who complained that her son had been sent to buy 10 tokens for a dollar at the Metro ticket office, only to end up in the more expensive underground token market.

"He was waylaid by some adult and sent to a pool hall on Benning Road, where they were selling them for a quarter apiece, no questions asked," Drayson said. "The mother was understandably upset that she had paid $2.50 instead of $1."

Metro Transit Police discovered the tokens had been stolen from an organization that is allowed to buy them in bulk, Drayson said, and that particular scam was ended.