The Prince George's County Board of Education voted last night to toughen rules on drug and alcohol abuse in the schools, requiring expulsion coupled with certified drug rehabilitation treatment before a student can be reinstated after a second offense.

Under the existing Code of Student Conduct, initial posession or use of controlled substances such as marijuana, heroin or barbiturates required a 5-day suspension while a second offense carried a long-term suspension, usually of one semester.

Under the resolution approved unanimously last night, alcohol possession will be treated like possession of other drugs and a third offense will bring expulsion that only the school superintendent can overrule.

The resolution was sponsored by board members Angelo Castelli and A. James Golato. Castelli, the only school board member with a contested reelection bid this fall, said students who use drugs are "playing with something that will eventually ruin your mind and your life."

Dr. Helen B. McAllister, county health officer, strongly supported the new rules. Students "need to have it strongly impressed upon them" that drug abuse "is a no-no. The health department will do everything it can to help you and back you up," McAllister told the board.

Last year 781 Prince George's students received short-term suspensions for alcohol or drug use; 94 got long-term suspensions for second offenses. The number of drug-related suspensions by itself declined from 954 in 1979 to 556 last year, while alcohol-related suspensions rose slightly during the period.

The new rules, which go into effect when school opens Aug. 30, are a revision of an original Castelli proposal that called for simple expulsion for weapon or drug possession. A modified version of the tough weapons regulation was passed in July.

Board members listened to testimony from several parents of drug-abusing students who praised the action. Many principals also attended the session, one of whom, Frank Tracy of High Point High, told the board: "There's no place for drugs in our high school. I have been to too many funerals and preached too many eulogies."