POTOMAC, Md. (AP) _ The parents felt reassured. By renting a bus to take their teen-agers to the Churchill High School homecoming dance, they thought they had guaranteed their children's safety.

What they hadn't anticipated was that the bus would make one extra stop - to an unsupervised keg party where the beer flowed freely.

Or that police would raid that party as part of a new, aggressive campaign against underage drinking.

Suddenly, they found themselves blamed - on the front page of The Washington Post - for encouraging their children's drinking, in a story headlined, ''Parents Hire Buses for Teens' Beer Party.''

Montgomery County is an affluent suburb of Washington, where teen-agers often drive around in their own cars. Some have busy parents who don't keep close track of them.

Now, these parents - like parents everywhere who grew up in the liberated '60s - are being forced to think seriously about the problems of teen-agers and alcohol, and about their own responsibilities.

Experts say the consequences of teen-age drinking are graver now than ever before. People often make bad decisions under the influence - and bad decisions can have dire consequences in today's world.

The Justice Department says a third of youths in long-term state juvenile institutions in 1987 were under the influence of alcohol when they committed their offenses.

And a 1990 survey found that nearly half of Massachusetts teen-agers were more likely to have sex if they had been drinking than if they were sober - and 17 percent used condoms less often after drinking.

''We didn't have metal detectors in schools. We didn't have AIDS. I didn't know 12-year-olds who had babies,'' said Pamela Beer, executive director of the Washington Regional Alcohol Program. ''It's a whole different world.''

That's what Beverly Shapiro, one of the mothers who rented the bus, says she is struggling with when it comes to her 15-year-old daughter.

''I can't tie her up and put her in a closet,'' said Shapiro. ''I tell her I'm not trying to spoil her fun. I just want her to survive these years.''

According to the National Highway Traffic Safety Administration, 3,000 to 5,000 people between the ages of 15 and 20 are killed every year in alcohol- related crashes. The message to teen-agers has been simple: Don't drink and drive.

Some teens think that means it's still OK to drink.

''I don't think it's a big deal as long as people don't drive,'' said Sarah Leshner, a 17-year-old Churchill senior who has been to several of the keg parties raided by police. ''They just drink to have fun.''

Ben Winters, president of the Churchill senior class, doesn't drink and has many friends who don't. But he is skeptical about the impact of any crackdown.

''You've got to be pretty stupid if you go to a school like Churchill not to know that kids have been drinking, because they've been drinking for a long time,'' Winters said. ''And they're going to keep drinking.''

Churchill parent leaders have been trying to drum their own message into the heads of other parents.

''Where are the parents when the kids are throwing these big parties?'' asked Dr. Ray Coleman, who helps run Churchill's School Community Action Team, which combats student alcohol use. Coleman is a pediatrician and the father of a Churchill junior.

''Where are they when the drinking is going on in their living rooms?''

In a letter after the bus incident, SCAT urged Churchill parents to put an end to unchaperoned parties with alcohol, set clear curfews, make sure teens are supervised when parents are away and check to make sure parties are supervised.

Police also have to shoulder some blame, says Sgt. Thomas Didone of the Rockville police. Not enforcing underage drinking laws in the past ''caused these kids to believe that they are entitled to drink alcohol,'' he said.

Instead of just showing their force once a year at prom time, police intend to become a presence in the schools. They will get to know teens, talk to them and teach them the facts about alcohol and the law.

They will also continue to show up at teen parties - and to give tickets to youths who break the law.

''This is a departure from what we used to do. We would respond to complaints of loud, disorderly parties, probably issue a warning and leave,'' said police spokesman Harry Geehreng. ''The message now is we're really serious.''