The D.C. public school system's 2,500 blue-collar workers voted overwhelmingly yesterday to join the International Brotherhood of Teamsters, dumping their current union, the American Federation of State, County, and Municipal Employees, after 15 years.

The Teamsters, who last year won an election to represent 2,400 D.C. prison system workers, now represent nearly 5,000 city employes and plan to expand their organizing efforts to sign up thousands more who belong to other unions. With yesterday's loss, AFSCME, the city government's largest labor union, lost about 25 percent of its membership among city workers.

The Teamsters won five separate elections yesterday among the school workers, carrying strong majorities of the cafeteria workers, bus drivers and warehouse workers, custodians, building engineers and cafeteria managers.

In the mail-ballot vote, counted yesterday at the offices of the American Arbitration Association, the Teamsters won 68.5 percent of the votes cast by nearly 1,400 workers.

"This vote shows that people really want strong representation," said Phillip Feaster, president of Teamster Local 639 in Northeast Washington. "Some of these people work under conditions that are horrendous, and they want the strongest union they can find."

AFSCME, which has represented the school employes since the early 1970s, before unions had full legal bargaining rights, had mounted a strong campaign to combat the Teamsters, hiring a firm to conduct opinion-polling among workers and using telephone banks and direct-mail leafletting to try to woo them back to the AFSCME fold.

"We are disappointed by today's results. We were proud of the solid gains we had recently negotiated for pay, bonuses and other issues" for school workers, AFSCME organizer Pete Moralis said in a statement. "AFSCME intends to deliver the best possible service to other District of Columbia employes who are members of the union, and to fight for improved contracts."

AFSCME's statement reflects concern about further Teamster "raids" on its membership. The Teamsters, who do not belong to the AFL-CIO, have been successful in a number of so-called raids on AFL-CIO unions such as AFSCME and the American Federation of Government Employees, which previously represented D.C. Corrections Department workers. The Teamsters have "no-raid" agreements with dozens of AFL-CIO unions in which each side agrees not to invade the other's turf, but have no agreements with AFSCME and AFGE, two of the District's largest unions.

The Teamsters have already begun organizing drug and alcohol counselors at St. Elizabeths Hospital, and plan to recruit hundreds of employes at D.C. Youth Service Administration facilities, and elsewhere among the 30,000 D.C. government employes.

School employes have had "deep frustrations toward the school system, and they took it out on the union," said an AFSCME official who asked not to be identified. "The school board has been a tough -- and needlessly tough -- employer, and workers take out the frustrations and blame the union."

School workers have long complained about understaffing, poor management, harsh discipline and lack of adequate supplies. Some custodians, for instance, said they are forced to buy cleaning supplies with their own money because the schools don't always provide it, and custodians can be disciplined for failing to clean properly.

School workers earn from $4 to $14 an hour, and pay has not been a major issue in the union election.

Corruption among Teamster officials was also a relatively minor issue, according to both sides. Teamster President Jackie Presser last month became the fourth Teamster chief indicted of the last five, but officials said the criminal image that has plagued the union was rarely mentioned by workers during the year-long campaign.