With political pressure building all around, the movie industry came to the White House yesterday with a plan of its own for limiting adolescents' exposure to violence: Teenagers interested in seeing an R-rated movie will now have to produce a photo ID first.

Representatives of companies that own two-thirds of the nation's movie screens said they would begin strict enforcement of the policy immediately, vowing that it will become much tougher for young teenagers to sneak into theaters where they don't belong. It's the latest reaction to President Clinton's request to combat youth violence in the wake of April's school shootings in Littleton, Colo.

Industry analysts privately questioned whether the new policy will do much to keep teenagers from watching lusty or blood-drenched movies, let alone prevent some future shooting spree. Nevertheless, executives clearly have decided it was important to redouble efforts to police the rating system in an environment growing increasingly hostile to their industry.

The movie industry has had a rating system in place since 1968, requiring that viewers of R-rated movies be at least 17 years old or accompanied by a parent or an adult. But in a White House ceremony yesterday, cinema owners acknowledged that children under 17 sometimes get tickets to R movies, which feature explicit sex and violence.

"Too often children do get past the ticket counter unescorted and under-age," Clinton told a gathering of PTA parents and their children in the Roosevelt Room. He said the theater owners' decision is part of a "national grass-roots campaign to prevent youth violence, to give our children the childhoods they deserve."

The announcement left several questions about the new campaign's likely effectiveness. It's voluntary, with no penalties for employees who let underage patrons into R movies. Moreover, one-third of the nation's movie screens are not covered by the agreement, nor are video rental outlets.

Mike Thompson, 19, a Rockville resident and college student, said he doubts the new policy will do much good. "Kids can always find a way to get in and see the movie they want," he said yesterday as he stood outside Tower Records in Rockville. "It's not going to work. And, if they can't get in to see the movie, they can still see it when it comes out on video."

Also, movie theater representatives are not sure how they will prevent youths from legally buying tickets for a PG or PG-13 movie, then slipping into an R-rated show in multi-screen theaters that typically have only one ticket-taker.

"We're going to do the very best we can," said William F. Kartozian, president of the National Association of Theatre Owners. His group represents more than 20,000 movie screens nationwide, or 65 percent of the nation's total.

"All of us, I believe, have had our sensibilities heightened since the recent tragedies at Columbine and in Georgia, and we as theater owners feel our responsibilities as keenly as does anybody," Kartozian said at the White House, where he shared the podium with Clinton. He was alluding to the Colorado high school where two teenage boys killed themselves and 13 others on April 20, and to a Georgia school shooting in May that wounded several students.

Liz Greene, spokesman for Blockbuster Inc., the nation's largest video rental retailer, said it is difficult for underage children to rent R-rated movies. Patrons must be at least 18 to get a membership card, she said, and children can't rent R-rated movies or M-rated video games with a parent's card unless that parent gives permission when joining the club. Other major rental outlets have similar policies, said a spokesman for Video Software Dealers, an industry trade group.

Since the Littleton shootings, Clinton has called on Congress to toughen gun restrictions, and he has asked the entertainment industry to stop marketing violent products -- including movies, records and video games -- to youths.

In yesterday's ceremony, Clinton said: "For rating systems to work, they must also be enforced, not simply by watchful parents, but by retailers at the point of sales, and theater owners at the multiplex. . . . When you drop them off, you shouldn't have to worry about your G-rated kids getting into violent or suggestive R-rated movies."

Yet some theater managers say it is the parents who are the biggest problem. "A lot of times the parents will buy the tickets and send them in" to an R-rated movie, said Kgosana Mangoaela, manager of the Loews multi-screen cinema at Wheaton Plaza. "But we don't let them in. A lot of times the parents get upset."

William Galston, who has written about youth violence as director of the University of Maryland's Institute for Philosophy and Public Policy, called yesterday's announcement "a modest but significant step." Galston, a top domestic aide to Clinton from 1993 to 1995, said, "A pretty strong link has been established between violence in films and on television on the one hand, and the propensity for youthful violence on the other."

As in last week's announcement to launch a study into Hollywood's marketing practices of violent movies, entertainment industry executives privately grumbled that yesterday's announcement seemed largely a public relations gesture that would have little practical meaning.

"We have always had this policy [of checking IDs for R-rated movies], and we will continue to enforce it and to look for better ways to enforce it," said Marc Pascucci, a spokesman for Loews Cineplex Entertainment, a nationwide chain of about 1,800 screens. The chain issued a statement saying it has a "longstanding policy" to "strictly adhere" to the industry guidelines.

Speaking on background, another theater executive said the White House move was a "smoke screen. . . . It's not like there's this overall malaise of kids sneaking in left and right. We're doing our part. This will blow over when the next issue is serial killers."

Staff writers Manuel Rivas-Perez and Sharon Waxman contributed to this report.

CAPTION: The president jokes with Douglas McWilliams of Nashville at White House event where movie industry announced tighter rules on admission to films.