UNIONDALE, N.Y. -- October is the time for homecomings and Halloween parties, but the 489 seniors at Kellenberg Memorial High School are already worried about their prom -- the one they are not going to have.

Fed up with the revealing evening gowns, flashy tuxedos, stretch limos, alcohol, drugs, sex and rowdy house parties that have become an increasingly common part of the dinner-dance scene across Long Island, Kellenberg administrators canceled the prom this year as a way to end the excesses that precede and follow it.

''We watched a pattern develop,'' said Brother Kenneth M. Hoagland, the principal of Kellenberg, a Roman Catholic school. ''Twenty years ago, seniors went to the beach after the prom and then to someone's house for breakfast. From that, it's turned into a weekend-long orgy that every year has become incrementally more excessive.''

The school sent parents a letter in March outlining objections to the blowout prom weekend. Calling it ''an exaggerated rite of passage that verges on decadence,'' the letter, signed by 11 administrators, said that spending up to $1,000 on formal wear, limos and after-prom parties was wasteful. It contended that a ''booze cruise'' and the rented party houses in the Hamptons were opportunities for illegal drinking and sex. And it said the school, fearing legal liability, could no longer be responsible for what might happen.

The warnings fell on deaf ears, Hoagland said. Last spring's prom was replete with all the extravagances of previous years, so he and the school president, the Rev. Philip K. Eichner, announced in September that they had no choice but to throw up their hands and cancel the prom for next spring. The prom, they wrote, ''is so much beyond our control that it is mere tokenism to put our name on it.''

Officials from the 10 other Catholic schools in Long Island's Nassau and Suffolk Counties said they, too, worried about the bacchanalia and had become more vigilant about warning parents that they risked arrest by permitting underage drinking. Sometimes the warnings work, sometimes not, but so far no one else has taken the next step to cancel the prom.

Sister Jeanne Marie Ross, the principal of Sacred Heart Academy, an all-girls school in Hempstead, said that last year she began talking to parents every chance she got, persuading one set of parents to chaperon their child's post-prom celebration at a Hamptons summer house.

''If these parents didn't rent the houses, the kids couldn't go there,'' Ross said.

This year, with eight months to go, Ross already has her ear to the ground. Sacred Heart, with just 219 seniors, is half Kellenberg's size -- all the better to catch the latest news about prom plans, she said.

''I think a call from the principal might scare some parents, but all we can do is hope we've talked about it enough that they will make good decisions,'' she said.

Kevin McBride, principal of St. Mary's High School in Manhasset, said he continued to warn parents of the potential dangers, but would not cancel the prom without at least trying to get their attention one more time.

''All schools are doing soul-searching concerning the proms,'' he said. ''There's considerable pressure on the kids because the prom looms large in their imagination, and parents want leverage over their own kids, so we hope a letter from the school will help them say no to all the excess.''

But Kellenberg officials said the time had come for more drastic measures. They called the problem a Long Island-wide phenomenon, involving not just Catholic but also public schools, worsened by some parents' willingness to bankroll the extravaganza.

''We felt that what the prom had become went against the moral and spiritual lessons we were trying to teach their children,'' Hoagland said. He said he worried most about parents who play host to cocktail parties before the prom and keg parties afterward, or pay thousands of dollars to rent a house in the Hamptons where unchaperoned teenagers hold raucous parties.

Those notorious prom houses so angered neighbors and local officials that the Southampton Town Police Department sent a letter to every high school on Long Island last year, warning that it was shutting down parties and arresting anyone who had broken a law.

Three Kellenberg seniors are hoping they can save at least a vestige of the celebration. They banded together to lobby for an alternative event that is more staid and better chaperoned. School officials said they were open to ideas.

Their tentative idea is for a carnival-like field day and barbecue, followed by a semiformal dinner-dance in the school, rather than a formal affair in a catering hall, the night before graduation.