Fairfax County school students may lose three days of spring vacation next year because of a Virginia General Assembly bill that would make all schools in the state begin fall classes after Labor Day.

The bill, which was adopted after intense lobbying by the state's tourist industry, has not been signed by Gov. Gerald L. Baliles. It would affect 68 percent of the state's public school students, including those in Fairfax, Loudoun and Prince William counties, where classes traditionally have begun before Labor Day.

Most of those systems have begun planning for the later opening, on the assumption that the governor will sign the bill.

The Fairfax School Board is scheduled to vote April 17 on two possible calendars for next year and the 1987-88 school year. Both calendars would begin school Sept. 2 next year, the day after Labor Day. One proposal would preserve all the current student holidays, hold high school graduations June 15 and end classes June 18.

The alternate recommended by Assistant Superintendent Warren Eisenhower's office would shrink the size of the spring break and change the scheduling of two teacher work and training days so that graduation can be held June 9. Classes would end June 12, the same day as this year.

Under the recommended calendar, students would get a spring break April 15-17 -- the Wednesday through Good Friday before Easter. Usually, spring break is a full week plus Good Friday. The two-day break between the second and third trimesters would be shortened to one day, and the teacher work day held the day before school ends would be held the day after classes ends.

Fairfax County School Superintendent Robert R. Spillane has complained that the later start will cost the system hundreds of thousands of dollars in extra pay, and some School Board members are concerned that absenteeism will rise if classes are pushed into late spring.

Fairfax officials say the revised calendar will cost them $80,000 next year to pay football coaches to come to work a week before other school employes because athletic league regulations require 20 days of practice before the first game, and the football schedule already has been set.

In addition, officials anticipate that more teachers will leave school before the end of the year to attend summer school, requiring them to hire substitutes.

Loudoun County schools, where a new calendar was adopted to comply with the bill, will begin Sept. 2 next year and end June 8, according to school spokeswoman Molly Converse. No holidays will be eliminated, she said.

In Prince William County, the School Board will vote Wednesday on a proposed calendar that would begin classes Sept. 2 and end them June 16, according to school spokeswoman Kristy Larson. The calendar will retain the current holidays.

Arlington County, Alexandria and Falls Church schools traditionally have opened after Labor Day.