Virginia Gov. Gerald L. Baliles signed into law last night a controversial bill prohibiting the state's public schools from opening before Labor Day, a measure strongly backed by the state's tourism industry but opposed by many local school boards.

The law, which affects about 68 percent of the state's public school students -- more than 175,000 of them in Northern Virginia -- means that state students will start the school year later and could stay in class until mid-June, officials said.

The bill's approval also means that all public schools in the Washington area will open after Labor Day. Public school students in the District and suburban Maryland are scheduled to start the next school year Sept. 2, the day after Labor Day.

The school bill was among more than 100 bills that Baliles was to act on before a midnight deadline. Baliles, who completed a hectic trip to Washington yesterday, rushed back to Richmond with more than 80 bills crammed into his briefcase and was working late into the night to meet the deadline.

Baliles declined to sign a controversial bill that would add Virginia to the list of Southern states holding presidential primaries or caucuses in the second week of March 1988. The governor was preparing to send the bill back to the legislature, which meets in a one-day veto session April 16, with an amendment that would address his concerns about the primary's impact on nominating individuals as delegates and for local and state offices.

The bill, pushed by Democrats over the objections of Republican leaders, is part of an effort by Southern party leaders to play a larger role in their national party. The governor's amendments will "clarify calendar dates involving the time" the political parties can act, a Baliles spokeswoman said.

Other bills signed by Baliles in recent days include a measure outlawing the sale or possession of cigarettes or other tobacco products for persons under age 16. The measure includes a $25 fine. Another bill signed would make it more difficult to bar group homes for the mentally impaired from residential neighborhoods.

Most of the measures take effect July 1.

Baliles also signed bills creating separate cabinet-level secretaries for economic development and the environment, fulfillng a campaign promise. A bill limiting the height of so-called high-rider trucks and other vehicles beginning in January also was signed.

But it was the Labor Day school bill, dubbed the "Kings Dominion Relief Bill" by critics, that has the most immediate effect on large numbers of Virginians, state government officials said.

Many school boards, including Fairfax, Prince William and Loudoun counties, had objected to the restriction on when schools could open, but all have begun to adopt calendars reflecting the later opening. Officials said it would add costs, and expressed concern that the state was interfering in a local school decision largely to satisfy the tourism industry.

Arlington and Alexandria schools currently open after Labor Day.

"The local control of public school calendars has long been the responsibility of local school boards," Loudoun County wrote Baliles in hopes he would veto the measure.

Legislators from Northern Virginia split on the issue, with many saying parents supported the later opening day while teachers, other school officials and the Virginia Education Association backed local control and the early start of classes.

"It's going to be okay next year [when Labor Day is early], but on the years when Labor Day is late, it's going to throw the school year late into June," said Molly Converse, a spokeswoman for the Loudoun schools.

A Prince William official said state School Superintendent S. John Davis in a recent memo warned local school systems to expect to begin school in the fall on Sept. 2, the day after Labor Day. The law is effective for two years.

The measure was supported by the state's tourism industry, which wanted to continue to attract Virginia families as well as make sure local teen-agers are available for entry-level, seasonal employment.