Best known for his off-the-wall antics in movies like Saving Silverman and High Fidelity, Jack Black probably isn't someone you'd want molding your kid's mind. Even so, he plays a substitute fifth-grade teacher in his newest flick, School of Rock (opening Friday). Hollywood's stage mothers can breathe a sigh of relief, though, since he didn't try to impart any slacker ideals to his young co-stars.

"I wasn't really a teacher," Black tells TV Guide Online. "In between scenes, we were just sort of compatriots. We were all in a platoon, on a mission, and they were rockin' as hard as I was. We would just hang out and fart around and just try to stay loose."

Better yet, Black — who pens pretty raunchy lyrics for his real-life band Tenacious D — cleaned up his crass vocab whenever little ears were on the set. "I dropped no F-bombs," the 34-year-old manchild recalls, with a tinge of regret. "I brought the rock, but I didn't want it to be a softer, cuddlier JB. That actually presented a challenge, because I had to make up for it by adding extra intensity — to communicate the F-bomb with my face, without ever saying any cuss words."

While Black watched his language, he was hardly an angel during Rock's shooting. "One time, I cut a really big fart," he shares. "It was really rude of me. But I thought about this one time I was in a movie with Jim Carrey; I had a really small part in The Cable Guy. I remember [during one scene], he was laying on the ground and they said, 'Action!' And he let out the loudest fart you've ever heard. Then, he just continued with the scene, until they said 'Cut!' and everyone just broke down, laughing so hard.

"I remember not laughing," Black adds, "just going, 'There is a lesson in this. He does not give a s--- what people think, and that's why he's so f---ing funny, because he is that free.' It sounds stupid that I learned a lesson from his giant fart, but [my fart] was a bit of homage to the master."