As the nation's educators prepare again to address the issue of what teen-agers know, the Motion Picture Association of America is worried about what they watch, particularly on video. At the annual Video Software Dealers Association convention last month in Las Vegas, MPAA President Jack Valenti issued a formal request that video dealers enforce the MPAA movie-rating system when renting cassettes to minors.

As is the case with theaters, participation is voluntary -- otherwise, the code would not be constitutional, Valenti says -- but the MPAA chief contends that if dealers go along, it might prevent the enactment of harsher mandates "written by legislators who don't know {the video} business." The time is right for such "voluntary self-regulation," Valenti told the VSDA membership, because "state legislatures are looking over our shoulder" at the ease with which kids can sometimes get hold of adult videos. Besides, he added, "It is both enlightened and prudent to sense public discomfort in advance and then press ahead to do something about it."

Valenti's request will have no impact at the Washington area's two leading video rental chains, Erol's Video Club and Video Place, both of which have longstanding policies that no one under age 18 is eligible for membership. Erol's assumes parental consent when teen-agers use their parents' membership cards. "Whoever has the card can check anything out," according to a company spokesman, although store personnel are instructed "to ask the kid if he's old enough."

Parents who join the Video Place club can indicate on the backs of their membership cards whether their children should be allowed to rent R-rated movies, but Frank Barnako, founder of the 11-store chain, says that most parents allow their teen-agers to rent whatever they want. Although the policy arose from concerns voiced three years ago by a Vienna zoning board, Barnako says he senses little of the "public discomfort" Valenti cited. "I get customer complaints every other week," he says, "but I have never had a customer complain about this to me."

Teen Trash

Teen-agers for whom R-rated features are off limits will soon be able to look into the low points of their parents' cinematic heritage with Teenage Theatre, a line of 1950s and '60s B movies due this month from Rhino Video. Included in the first batch are Roger Corman's 1957 musical drama "Carnival Rock," the 1956 girl-gang drama "The Violent Years," the 1960 boy-gang drama "High School Caesar" and "Teenage Confidential," a 60-minute compendium of clips from teen films of the era, period newsreels and government films on the "Juvenile Delinquency Dilemma." Vintage vamp Mamie Van Doren, whose tell-all memoir "Playing the Field" is among the fall's literary offerings, serves as the series' hostess, introducing each film. The $24.95 tapes are available only in VHS, unlike Rhino's best-known title, "The Wit and Wisdom of Ronald Reagan," a $7.95 blank tape.

More Than a Movie

"See 'Platoon' soon" is the new battle cry of HBO Video, which has put the catch phrase at the center of its upcoming television ad campaign for the Oscar winner's October video release. Several of the six 15-second spots seek to appeal to something other than viewers' taste in movies. "It's not too late to do something about the Vietnam War. See 'Platoon' and understand," intones an offscreen announcer in one. Another presents a monologue from a worried-looking wife: "When he came back, he didn't talk about Vietnam. But every night I knew he was dreaming about it. Then I saw 'Platoon,' and I saw what he'd been through and what he couldn't talk about." HBO plans to spend $1.5 million advertising the tape on television, a record for a video priced for the rental market -- "Platoon" will sell for $99.95 .

Autumn Action

Rather than team up with a national news organization, as previously announced, MPI Home Video now plans to produce a Bernhard Goetz video on its own, using footage from free-lance journalists and interviews conducted especially for the video, according to MPI Vice President Jaffer Ali. The company has access to the taped confession, but not on an exclusive basis as reported last week: The tape became part of the public record when it was screened in open court earlier this year, Ali says. But all this will be for nothing if Goetz's attorney, Barry Slotnick, has his way: He has vowed to file an injunction barring the distribution of the tape ... Warner Home Video is calling it a "Walking Weapons Trio": a group of action films scheduled for release on video next month. The recent Mel Gibson-Danny Glover blockbuster "Lethal Weapon" leads off, along with two '70s martial-arts treasures: Tamara Dobson as the title character in "Cleopatra Jones and the Casino of Gold" and "Enter the Dragon" star Jim Kelly as Black Belt Jones in "Hot Potato." All three should arrive in rental stores in late October.