Alexandria students will not participate next year in Fairfax County's new regional science and technology high school, the city's School Board decided last night.

"We have nothing to lose by waiting," said Superintendent Robert W. Peebles, echoing the board's unanimous decision to observe how the high school operates next year and decide later whether to send students there in the 1986-87 school year.

Tuition and transportation costs and the potential loss of some of Alexandria's "best and brightest" students were cited as reasons for not participating next year. For each student accepted Alexandria would have had to pay $4,500.

Of the seven Northern Virginia school systems that Fairfax invited to participate in the so-called "magnet" school, only Alexandria and Arlington decided against doing so.

Corporate sponsors have donated over $1 million for computers and laboratories for what will be the Thomas Jefferson High School for Science and Technology in Annandale. The school, now Jefferson High School, will draw students next fall from Loudoun and Prince William counties and the cities of Falls Church, Manassas and Manassas Park.

"As far as Fairfax County is concerned, it's not critical that they Alexandria and Arlington will not join," Fairfax County School Superintendent William Burkholder said yesterday. Burkholder added that he did not believe the high tech school would absorb most of the region's top students and emphasized it will only educate students specializing in mathematics and science.

"Hair splitting" is what Burkholder termed some other criticisms raised over the new school.

Alexandria School Board member Timothy Elliott said he was concerned that students from Alexandria's private schools might take up some of the 20 seats allocated to Alexandria if the board approved participation.

The number of students selected from each district is calculated according to the percentage that district's school population represents in Northern Virginia. Alexandria, with 5 percent of Northern Virginia's public school enrollment, could send 20 students to the new school. Elliott argued if private students are eligible they should also be included in the formula.

In other action, the board voted unanimously to scrap the $1.7 million bid on a new riverfront boathouse for the T.C. Williams High School crew.

The City Council rejected the board's request for an additional $775,000 Tuesday night to build the new crew facility. With no more than the $956,000 allocated by the council last spring, the board voted last night to initiate fresh discussions on the site and design of a less expensive boathouse.