Emma Tierney and her friends blew off Unionville High School's junior-senior prom last year, but they never for a minute considered skipping the after-prom party.

"It's just a lot of fun," Tierney said.

More than that, it turned out to be one of the most exciting days of the 17-year-old senior's life: She won the evening's grand prize, a 2002 Honda Civic.

Though proms are still a memorable rite of passage for most high schoolers, the after-proms are boldly becoming bigger, showier, and more fun than the main event.

For some students, after-prom is like a delicious midnight snack eaten on the sofa, more relaxed, tasty, and comforting than the stiff sit-down meal that preceded it.

The parent-run parties start when the formal dinner-dance ends and run through the night, with enough games, entertainment, and prizes to occupy the kids until sunrise. Seniors shed gowns and tuxedoes for shorts and flip-flops and a sleepless 24 hours or so.

The purpose is to keep the go-all-night adolescents off the streets and away from booze. And if they have to be bribed with cars, iPads, laptops, cash, magicians, hypnotists, laser tag, casino rooms, food - lots and lots of food - parents are happy to do it.

"I want the kids to have something fun to do that is also safe," said Jim Jenkins, a parent and computer "geek" who last year made a replica of City Hall for Haverford High School's BLAST, the after-prom, which this year is Friday. When he was in high school in Maryland, a senior died in a car accident after the prom.

"That can ruin a community," he said. "We don't want that to happen."

Prom season can indeed be deadly. According to the National Highway Traffic Safety Administration, between 2006 and 2010, May had the most alcohol-related traffic fatalities for 16- to 19-year-olds during the school year.

In 2007, a Little Flower Catholic High School senior, Lacey Gallagher, was en route with six friends to a post-prom Poconos weekend when their car hit a concrete divider and flipped at 3 a.m. Gallagher, the driver, died.

"We don't want to be a part of those statistics," said Sue Nolan, chair of Penncrest High School's AM Prom, one of the flashier events, with a $40,000 budget and 100 volunteers.

The theme of the party is kept a secret to enhance the "wow" factor when kids first lay eyes on their barely recognizable schools. Last year, it was "The Movies," with a parent-built movie marquee in the lobby. The Star Wars gym had laser tag; the courtyard featured miniature golf from Happy Gilmore; the cafeteria looked just like the Hogwarts dining hall, complete with suspended candles over long tables; and the rest area had a tree of life from Avatar.

As Radnor High School's after-prom chair, Pattie Booker, said: "It's like being at an amusement park."

And winning all the carnival games. The parties are awash in giveaways, including such teen-pleasing items as gift cards to Wawa, movies, and Bed, Bath & Beyond, as well as computer tablets and TVs.

At Penncrest, each of the 362 seniors will go home with a gift, some with three or four. Nearly every student attends, even those who don't go to the prom.

"That's upward of 500 teenagers [including dates] that we take off the streets and provide a fun, safe way to be together after prom," Nolan said.

To get such high attendance, the parties have gotten more elaborate - and expensive. Starting in September, an army of parents begins to cut coupons, look for sales, hit up businesses for donations, and hold fund-raisers to boost the budget.

Here's the thanks they get: Penncrest's prom is Saturday, meaning the after-party starts as Mother's Day begins. Instead of sleeping late and waking to breakfast in bed, moms will be supervising hundreds of sleep-deprived teens and dragging themselves home at 6 or 7 a.m.

"We're not thrilled with it," Nolan said. "The way we look at it is, it's for our kids, and we're mothers, and we're happy to do it."

Unionville is like the Oprah Winfrey of after-prom parties - somebody gets a car, which is donated by Scott Honda of West Chester and tantalizingly displayed at the school the week before the big day.

No wonder about 600 teens are expected to show up May 19.

Tierney, who was a junior when she won, said 10 students were called to the stage at the end of the night. Each got a small package with a Matchbox car inside. Hers had a key under the car.

"It didn't register. The girl next to me said, 'Emma, you won,' " she said. The black Civic had 100,000 miles on it, but "they run for a long time," said Tierney, who drives it to school every day.

Besides the grand prize, the party features airbrush tattoos, a hypnotist, caricaturist, fortune teller, DJ, live bands, masseuse, even hairstylists to help girls remove fancy up-dos and touch up makeup.

Parents spend three days decorating "to the point where people walk in and they're not exactly sure where they are," said Michelle Montgomery, PTO co-president.

While students are still grinding on the dance floor at the prom, the schools open their doors to the community so the public can ooh and ahh at the handiwork. At Haverford last year, visitors were greeted with a volcano made by David and Colette Orehowsky.

The couple used a five-gallon bucket and five long pieces of wood for the base, covered it in brown and red burlap, ran red lights down the sides for lava, and installed a CD with volcano sounds inside.

And the Orehowskys didn't even have a child in the senior class. At Haverford and elsewhere, that's not unusual.

But parents can be too enthusiastic. At Radnor, the decorations got so over the top - one mother made a papier mâché airplane that looked as though it had crashed into a wall - that in recent years the decor has been tamped down.

"The thing had kind of rocketed out of control," Booker said. "Every class was trying to outdo the previous class."

For this year's June 8 event, most of the $22,000 budget will go toward entertainment and prizes.

Sometimes, the best prize is the simplest. Unionville hands out five $100 checks, and Haverford has something called a cash cube. Students stand inside an inflatable cube that blows money around and try to catch as much as they can.

Said Haverford's Jennifer McNicholas: "We try to make it beneficial for them to stay and have a good time."

Contact Kathy Boccella at 610-313-8123 or kboccella@phillynews.com.