In its second season, "21 Jump Street," a Sunday night series about baby-faced undercover cops, has become Fox Television's most successful show.

Pictures of the four young actors are pasted inside the school lockers of teen-age girls whose devotion recently pushed "Jump Street" into first place in its Sunday time period among teen-aged girls 12 to 17.

The Fox people will tell you, with understandable delight, that "Jump Street" is beating out ABC's "Disney Sunday Movie," "NBC's "Our House" and CBS' "60 Minutes" with that young audience, despite the fact that Fox Television reaches only 86 percent of the nation's viewers.

And they are also proud that public service announcements that follow each episode dramatically increase the number of toll-free calls going to assistance agencies.

It's possible, of course, that you've never heard of the "21 Jump Street" stars, whose job is to infiltrate the world of teen-agers to find out what's gone wrong. Johnny Depp, Holly Robinson, Peter DeLuise and Dustin Nguyen play the undercover agents; Steven Williams is their supervisor, Capt. Adam Fuller.

"I didn't want to cast names -- I wanted to cast talent," explained creator/executive producer Patrick Hasburgh. "Johnny Depp, he's got real talent. He came out here from Florida as a rock 'n' roll guitarist. Holly Robinson -- I didn't write any of these parts for a black woman, but I felt she had the most energy. She has done a great job. DeLuise is a giant talent. He is only 20 years old. When I was 20, I was throwing rocks at police. Steve Williams is a real solid, terrific actor on the set and in the show -- he's worked in the business for 20 years. And Dustin, our Christmas episode is going to be his story."

Next Sunday, "Jump Street" tells the account of Dustin Nguyen's real-life escape from Vietnam. In the show, Nguyen's parents and best friend are killed. "Dustin was very fortunate that his family got out intact, but otherwise the show is very accurate. It's in some ways exactly what happened. He's going to be playing himself at age 12, when America pulled out, running down the beach, being shot at by North Vietnamese ... "

Recently, Hasburgh, 38, took time out between shooting episodes in Vancouver to talk about the series that has Fox folks proud. "Apparently we are the number-one show in the country for teen-agers. I'm real proud of that ... We're doing real dramas for kids without wrapping them up neatly at the end. Kids like that, like to be talked to like adults. My concern is that I don't want to come off being pretentious. We're not trying to preach."

Hasburgh has a clear idea of what he wants "Jump Street" to be: "A show that parents don't want their children to watch," he said, only partly in jest.

"Fox originally wanted 'Jump Street' to be sort of a light, 'Happy Days' sort of show," explained Hasburgh. He disagreed with the brass, and for the most part, he seems to have won. "They're very young, in some ways inexperienced. They're hard to work with because they think they know more than they do," he said, then added: "But the relationship has worked very well so far."

Hasburgh and co-creator Stephen J. Cannell (officially Patrick Hasburgh Production in association with Stephen J. Cannell Productions) and their writers have tackled topics that may be controversial to parents: racism, date rape, sexual abuse, child porno movies, drunk driving, steroid overdose, teen-age pregnancy, interracial dating, incest, parental kidnapping, child prostitution, attacks on homosexuals.

After most episodes, one of the "Jump Street" stars does a public service announcement offering an appropriate hot-line telephone number.

"Last year we ran a show on sexual abuse -- the father sexually abusing the daughter -- and that night the hot-line got 4,000 calls," said Hasburgh. The average number of phone calls on that hot-line is closer to 900, he said. "Apparently with the one we ran involving pornographic movies, two porno rings in the midwest were busted ... Films {of that episode} are being used as training films across the country for police officers."

Hasburgh, who still writes some of the scripts and supervises the others, said he tries to "embrace those issues really straight-ahead and honestly ... I simply deal with something that interests me. I don't simply aim the shows at the teen-age audience per se. I assume that the teen audience is interested in what's in the newspaper every day. Date-rape. Confusion about what's right and wrong. Racism. That one I wrote last year during the Howard Beach situation and the network wouldn't let me put it on. I think it's a subject that's interesting, and I also think it's thought-provoking. If it makes people uncomfortable, great.

"I think there's been a breakdown in communications ... I think the fault of television recently, particularly in shows aimed at this audience, is that they have the need to finish the show neatly."

Like an increasing number of other productions, "21 Jump Street" is filmed in Canada. "At first, the dollar was much stronger. We could save 20 percent off the top," Hasburgh explained. Even after the dollar began to fall, he said, "I still think I would choose to do it here. It has rivers, city, mountains ... There are about 10 companies up here . . . It's Hollywood North. I see more people up here than I ever do in Hollywood."

Hasburgh likes Vancouver so much, in fact, that he is considering buying a house. He also likes the setting and the people and even the light, which he said is sometimes startlingly clear: "You don't have your basic three brown palm trees and oppressive gray-gold sky. This is a real place with real people -- they're all kind of nice, normal. They're still sort of thrilled with show business. It's sort of like America thinks it is."

Hasburgh grew up in Buffalo, N.Y., in a neighborhood that was apparently a cradle of creativity: Eight of his childhood acquaintances became producers, he said, "and five of us have series."

Hasburgh worked at Bethlehem Steel right out of high school, then went to Colorado where he taught skiing in the winter and served as a white-river guide and drove trucks in the summer. He also wrote free-lance articles, plays and screenplays -- on speculation -- while working on Inside Aspen magazine.

"I started getting into television in my late 20s, wrote quite a few television shows that didn't sell but that I used to showcase my work. I sold my first script when I was 30 years old -- Stephen Cannell bought it."

Of scriptwriting, Hasburgh noted: "It's become a specific art form. It's a lot like singing: Everyone thinks you have talent but you don't. It's hard to find people who can write TV effectively and well. Being in a television series is very, very hard. You put in 10, 12 hours a day six days a week. Every two weeks we make one movie, two hours long. To keep the average up is impossible. We had one real bad script -- it was about nothing. I think I'll run it during the Super Bowl."

Hasburgh became a writer and story editor on Cannell's "Greatest American Hero" series and was a story editor on "The Quest." In 1983 he helped develop and produce the pilot for "The A-Team," then served as writer and producer for the series' first year. "It was a relative hit, for some," he observed. He also served as co-creator and executive producer of "Hardcastle & McCormick," writing and directing many episodes.

Hasburgh said he has a new series for Fox that's "definitely set for next season. It's about a young black juvenile delinquent who while he was in reform school took a correspondence course and came out with a degree. It will have a black lead. The kid comes out of reform school, gets his conviction overturned, takes the bar, becomes an attorney and becomes a public defender ... It's a straight-ahead show about law, how the system works, not like 'L.A. Law.' It's called 'Return of the Prince' ... Prince was his name when he was a gang leader. It's tonally the same as 'Jump Street.'"