The Virginia Senate passed and sent to the governor today a bill that would prohibit public schools from opening before Labor Day, a measure strongly supported by the state's tourist industry.

The bill, opposed by many school officials, would affect about 68 percent of the state's public school students, including most Northern Virginia school systems.

"We love our school system, but we also love our tourist industry," said Sen. Edward M. Holland (D-Arlington), who supported the measure. It has been nicknamed "The Kings Dominion Relief Bill" by opponents citing the claims of the bill's advocates, who said the measure would be a boon to tourism and allow youths to hold summer jobs longer at theme parks, beaches and other businesses.

The bill, previously approved by the House, passed by a 24-to-14 vote with one abstention during a day in which the Senate also gave final approval to a bill that would ban children under 16 from buying or possessing cigarettes and tobacco products.

In the House, rebellious lawmakers again thwarted attempts by the Democratic leadership to approve a controversial Senate bill to weaken the state's conflict-of-interest act, a step likely to force the issue into a conference with the Senate. The House also delayed final approval of a mandatory seat belt bill until it can resolve differences over the measure with the Senate.

If Gov. Gerald L. Baliles signs the bill delaying school openings it would prohibit any public school from opening before Sept. 1 next fall. Alan D. Albert, a Baliles adviser, said today that the governor has not expressed an opinion on the bill.

A spokesman for state School Superintendent S. John Davis declined to comment on whether he would recommend that Baliles either sign or veto the bill.

Opposition to the bill came from school and teacher groups that said the measure favored business interests at the expense of education and infringed on the authority of local school boards.

"Education, not tourism, is the main purpose of schools," scolded Sen. Stanley C. Walker (D-Norfolk), in what for him was an unusually emotional speech.

Proponents called the bill "experimental," saying it would affect only the start of classes in 1986 and 1987. If the delay is to continue, the legislature would have to extend the law.

The bill would allow the Virginia State Board of Education to exempt local systems that can show "good cause" to open earlier.

"We have no business, no business whatsoever getting involved in the day-to-day operation of our school systems," said Sen. R. Edward Houck (D-Spotsylvania). "This is special interest legislation at its finest."

The bill's sponsor, Del. Alson H. Smith (D-Winchester), who watched from the Senate gallery, said he introduced the bill at the request of the Virginia Travel Council, a private group that promotes tourism.

He said a recent Virginia Commonwealth University study showed that parents overwhelmingly favored a later starting date for school.

"The institution of Labor Day . . . is fast disappearing," said Sen. Charles L. Waddell (D-Loudoun), who supported the bill, saying it would bring families together.

Sen. Wiley F. Mitchell (R-Alexandria) was the only Northern Virginian to vote against the bill.

Sen. John W. Russell (R-Fairfax), who previously opposed the bill but played a key role in committee to allow the measure to go to the Senate floor, voted today for the bill. He said he switched because of numerous telephone calls from parents supporting the bill.

"It will work a hardship on school boards, pupils and teachers alike," said Richard Pulley, a lobbyist for the Virginia Education Association. He said any gains for student employment in the fall would be wiped out if students have to stay longer in school in the spring to complete 180 mandatory school days.

Last night, Fairfax County School Superintendent Robert R. Spillane said the General Assembly's action "is not quite as simplistic as it seems." He announced at a School Board meeting that the next school year will start Sept. 2, the day after Labor Day, and noted that "the new rule alters grading periods, the final day of school, graduation dates and sports calendars. It is unfortunate at this point . . . but we'll try to do with as little disruption as possible."

The antitobacco bill approved by the Senate would impose a fine of $25 on persons who sell tobacco products to children under 16.

The children also could be fined for possessing tobacco products unless they expressly were delivering them to their parents, a provision similar to the law regulating possession of alcoholic beverages.

It would be illegal for minors to purchase cigarettes from vending machines but vending suppliers and merchants would not be held liable for such sales.

State school officials have sought the measure to help reduce smoking in schools and to discourage the increasing addiction of youths to smoking and the growing use of smokeless tobaccos.