FROM the “where-are- they-now?” department.: Arnold Horshack is back in school.

Only this time, he’s a teacher.

Ron Palillo, who played the wisecracking Horshack in the 1970s sitcom “Welcome Back, Kotter,” is now teaching acting to freshmen and sophomores at a charter high school in Palm Beach, Fla., according to reports over the weekend.

Palillo says he started working at the school this fall.

Things are a lot more serious than when he and John Travolta played kids on the TV show.

“You don’t see any Sweathogs in my class,” he told the