SAN JOSE, Calif. – Mariah Miller had already tried on – and rejected – dresses in Electric Blue, Lemon Drop, Purple Passion and Shimmer Moss, colors created to inflame the imagination of 16-year-old prom girls like her. By the time she arrived at Trudys formalwear store in Campbell, Calif., on Saturday, Miller and her four-woman prom posse had trudged through the changing rooms of prom shops in Walnut Creek, Concord and San Jose the previous weekend.

Her grandmother, Pam Tsouvas, survived that 12-hour march – but just barely. “We must have tried on hundreds of dresses,” she said, “and we found nothing.”

By “we,” of course, she meant her picky granddaughter, a junior at Liberty High School in Brentwood, Calif., where the theme of this year’s prom is “Midnight in Paris.” Miller seemed to know precisely what sort of evening wear was required for such an occasion – not that she’s ever been anywhere near Paris.

Into the life of every high school girl come a few big moments, and one of the biggest is the prom. As prom season approaches, picking out the perfect dress for the most important school dance of the year is the high school equivalent of preparing to walk the red carpet at the Oscars. It’s a rite of passage, a ritual that prefigures the selection of the wedding dress to come.

Stores such as Jessica McClintock do a thriving business selling prom dresses of their own design. But Trudys – a 36-year staple – claims to have the largest selection of prom dresses in Northern California, and the store has sought to widen its advantage with a time-tested gimmick: It won’t sell identical dresses in the same color to two girls going to the same prom.

“Yeah, I wouldn’t want anyone wearing the same dress I’m wearing,” said Hilary Willis, a senior at Livermore High School.

She was still waiting to be invited by a boy she had in mind, but was prepared to go with friends if it didn’t happen.

“I think he’s going to ask,” she said as she waited for her turn in one of the jam-packed store’s 19 fitting rooms.

In the run-up to prom season, the wait for a dressing room on Saturday afternoons can stretch to an hour.

“Our prom girls tend to sleep in,” said Trudys manager Debbie Azevedo. She’s responsible for enforcing a strict limit of four dresses per girl when there’s a waiting list – a rule being flouted by Miller with an impunity fueled by desperation and a 16-year-old’s anxieties.

“I don’t like shopping for dresses,” she said, and yet she couldn’t seem to stop herself. “You want to look good.”

Her grandmother was impressed with the store’s inventory, which included Spanx – an elastic hip flattener – and some accessories that Miller was stuffing into the halter tops of every dress she put on.

“If you need the boobs, they’ve got ‘em,” Tsouvas noted as Miller examined her profile enhancements in a three-way mirror. “Look at these puppies,” she said approvingly.

The store starts getting calls by the end of October about prom dresses that don’t actually arrive until January.

“These girls start school, and they’re already thinking about what they’re going to wear to the prom,” Azevedo said.

The dresses are a perishable commodity. By May, most are gone. “Designers don’t want to end up with a warehouse filled with dresses that will be out of season next year,” Azevedo said.

Prom dresses have gotten a lot more off the shoulder than they once were.

“The dresses are designed based on what celebrities are wearing, what’s hot in Hollywood,” Azevedo said. “When they see a celebrity wearing a plunging neckline, our prom girls want that plunging neckline. They want the very low, dramatic backs.”

Bare backs seemed popular at Trudys, although Willis insisted she wasn’t basing her decision on movie stars.

“I know what I like,” she said. “I don’t care what Cameron Diaz is wearing.”

Jennifer Protsman, a senior, bought an emerald green dress that set off her red hair during a special prom party at the store Friday night. Her dress was fairly modest, but she insisted if it had been otherwise, that should be her choice alone to make.

“How many other times in your life are you going to get to dress up like this?” Protsman asked. “We don’t mean to be provocative. This is our prom, and I think girls should be able to dress however they want to. People look at the way guys dress, and they’re never critical – ever. A guy can wear a pimp suit and no one would make a big deal about it. But everyone’s always critical of girls, and I just don’t think it’s fair.”

For some girls, shopping for the dress would be the high point of prom season.

“I think girls nowadays are more focused on what they look like at prom than the actual dance itself,” said Marina Schlaefli, a senior shopping for her third successive hot pink prom dress. “The funnest part is to get dressed up.”

Protsman agreed. “Once you get to prom, no one really cares that much how you look,” she said. “No one remembers what you wore.”