The prom was last month, but high school seniors in Loudoun County are still celebrating their impending graduation.

And this time, their parents are crashing the party.

They are hawking tickets, soliciting door prizes and decorating community centers for the drug- and alcohol-free event known as Project Graduation. Parents from each of the five public high schools are planning a party on graduation night lasting until roughly 5 a.m. the next day.

An estimated 40 schools in Northern Virginia are holding similar parties this year, Loudoun parents said, in a growing tradition designed to help steer graduates away from drunk driving.

"Safety is the top factor here," said Liz Boastfield, who is coordinating the party for Loudoun County High graduates.

Rules at all of the parties prohibit admission to anyone who is under the influence of alcohol or drugs. Seniors at the party who decide to leave the event cannot return.

But, Boastfield said, "behavior is never really a problem."

With the parties less than two weeks away, parents are furiously fund raising to pay for the henna tattoos, face-painting, caricaturists and other features that raise the cost of each Project Graduation party to as much as $25,000.

"The donations, in my opinion, have been fairly low," said Cindy McNanley, who is one of the organizers of the Potomac Falls High School party. Because Potomac Falls opened only last year, there is no reserve from the previous year that other schools typically use to help finance their events, she said.

Tickets are $50 to attend the Mardi Gras-theme party at Potomac Falls and could be raised to $60 in the last remaining week, McNanley said. More than half the graduating class of 253 has bought tickets.

Beyond ticket sales, McNanley launched a number of fund-raisers. A weeping cherry tree sale failed because the trees were about two feet shorter than advertised, she said. But supermarket scrip, which returns a portion of a customer's bill to the school, has generated $4,000 to $4,500 for the party fund.

McNanley said some Potomac Falls students said they won't attend Project Graduation because they're going to a "real" party instead, one she assumes will include alcohol. Anyone who shows up drunk at the Mardi Gras party will be detained out

side the building until a parent picks him or her up.

Despite the lack of illegal substances, the increasingly elaborate parties continue to draw students.

More than 170 graduating students at Loudoun County High School have paid $25 apiece to attend "Anchors Away '99," the parent-run graduation party at Ida Lee Park. The community center will be decorated to resemble a cruise ship, and Hawaiian shirts are encouraged.

Broad Run will hold its party, a "Spartan Mardi Gras," at the Ashburn Village Sports Pavilion. Park View seniors will throw their party, a "Celebration of the Century," at the school in Sterling.

Loudoun Valley graduates can attend a "Mexican Odyssey" at the Lovettsville Community Center. They will enter the party through a grass shack, check in at the Tijuana Room and compete for prizes at the Guadalajara Casino.

Priscilla B. Godfrey, who is organizing the Loudoun Valley party, said the parties have become creative to attract as many members of the senior class as possible and to keep them there until the conclusion at sunrise.

"Some students would come no matter what, even if there was just Ping-Pong in a basement," said Godfrey, whose daughter will graduate this year. "But if you want to get a critical mass of more than 100 kids, you have to really dress it up."

CAPTION: Loudoun County High student Keisha Washington, right, turns in a raffle ticket to parent Liz Boastfield, who is coordinating the school's graduation party.