Kelly O'Donnell took in a late-afternoon showing of the movie "South Park" the other day in Rockville. She found the raunchy, expletive-laden animated film "kind of stupid."

Which is a remarkable thing for Kelly to say. Technically speaking, the 15-year-old from Montgomery County shouldn't have seen the movie at all.

"South Park" is rated R, which means patrons under 17 aren't supposed to be admitted unless accompanied by a parent or guardian. Kelly was accompanied, all right--by a 15-year-old friend. Both got into Rockville's Regal Theater without a hitch. "I've never had a problem" getting in to see R films, she said. "I've seen a lot of them."

So much for good intentions. Under a plan announced last month by theater owners with President Clinton's endorsement, the movie industry was supposed to be getting tough on the Kellys of the world. Theater companies across America pledged to step up their enforcement of the movie rating code, putting the "restricted" back in the R rating. With the horrifying images of Columbine High still fresh and the media taking some of the rap, the industry vowed to check youngsters' IDs to make sure they weren't slipping into movies intended for older audiences.

And indeed, the signs of a crackdown are evident.

At the Bethesda AMC theaters on Wisconsin Avenue, that means bright-yellow notices have been posted on the front doors that warn teens, "Your ID will be checked at the box office AND at the door to the theater. There will be no exceptions or refunds." At the AMC Courthouse Plaza 8 in Arlington, cashiers wear huge buttons reading "Are you 17?" At the Regal in Rockville, usher Bruck Girma, 17, says, "We're being very strict about it."

It all sounds tough. In practice, however, it doesn't seem to take too much ingenuity or extra effort for the underage to make it into "South Park: Bigger, Longer & Uncut," the movie version of the cable TV series. The system, it seems, is only as strong as the willingness of busy ushers and cashiers to enforce it--which means intermittently and inconsistently.

Despite a more watchful policy at the Broadway Cineplex theater in Santa Monica, Calif., three 16-year-olds walked into the movie on Thursday after Elisa Choi, 19, agreed to buy them tickets. "I think it's kind of ridiculous to have to show ID," she said after she handed the tickets to the teens. "Movies are a time where you go out, you want to relax. As if they don't show enough bad stuff on television and in magazines."

Julius Onah and Emory Kristof, both 16, were turned away from one showing of the movie in Arlington, but quickly figured out a way in. "[A friend] took three other people's IDs [and] bought the tickets with those," says Emory, wearing a baseball hat and a mischievous grin. "Those IDs were from people who were over 17 and already bought tickets. Truth is, I've been seeing R-rated films since I was 4 and I think it's helped my maturing."

The boys were asked for their IDs by the ticket-taker, and they dutifully complied. Yet even though their school picture IDs didn't specify their ages, the pair was waved in.

Over at the Regal in Rockville, it was much easier. Three teenage boys holding tickets to "Wild, Wild West," a PG-13 film, were directed by a ticket-taker to theater 5 of the multiplex. But once out of sight, the trio veered left into theater 1, where "South Park" was about to start. They were among several unaccompanied youngsters for the 8 p.m. show on Thursday. "Nobody hassled us," said one of the boys, who declined to give his name."

The efforts of young people to get into "South Park" have a certain life-imitating-art-imitating-life quality. The movie is itself about children who sneak into a vulgar, R-rated movie (by paying a homeless man to buy tickets for them) and the disproportionate reaction of their parents (they become so riled by their kids' foul mouths that they persuade the United States to declare war on Canada, where the movie-within-the-movie was made). There's profanity aplenty, plus a little religious bigotry and some rather explicit sexuality involving Saddam Hussein and the Devil.

The movie's creators, Trey Parker and Matt Stone, had to persuade the Motion Picture Association of America's rating board not to give the film an NC-17 rating, which would have prevented anyone 17 or under from seeing it in the theater even if accompanied by a parent (the less restrictive R rating enables those 16 and under to see a film with their parents).

Indeed, the movie is in part a satire about the MPAA ratings system and the TV-program-blocking V-chip (one character has a V-chip implanted in his head that shocks him each time he utters an expletive).

For teens, the R rating itself may be an enticement. Some experts, such as Joanne Cantor, a University of Wisconsin communications professor, have suggested that the ratings create a kind of forbidden fruit, challenging young people to see what their parents disapprove of. Since teens are one of the most loyal audience segments, Hollywood gears many R-rated films--such as the "Scream" series and the forthcoming "American Pie"--toward them.

Even so, the MPAA says parents have consistently given high marks to the system. "Survey after survey shows that parents, who are the [people] we created this system for, find it to be useful in making decisions for children," said MPAA spokesman Richard Taylor. "That has always been the goal of this system."

The problem, some parents say, is the porous enforcement of the system--and the movies themselves. Outside the Arlington theater, John Smith, 44, an environmental engineer from McLean, stands with his 10-year-old son, Andrew, lamenting "South Park." "It's not a family values movie," he says.

Like many parents in front of the theater, he falls into the group that believes R ratings and tight enforcement are good for the children, the movie industry and society in general.

The Smiths won't be taking their kids to see any R-rated films. Instead, his son just returned from "Austin Powers: The Spy Who Shagged Me," which is rated PG-13.

Staff writer Sharon Waxman contributed to this report from Los Angeles.

CAPTION: An 11-year-old and his mother buy tickets at AMC City Place in Silver Spring to see the R-rated "South Park," top, which pokes fun at the movie ratings system.

CAPTION: At Hollywood's Galaxy Theater, a sign warns that moviegoers' IDs might be checked at the box office or theater door.