RON PALILLO . . . who played high school student Arnold Horshack on the '70s sitcom "Welcome Back Kotter". . . died yesterday morning of a heart attack.  He was 63. "Kotter" ran from 1975 to 1979 and as you probably know, it launched the career of JOHN TRAVOLTA.  Here's a clip of "The Great Debate".  See if you recognize a well known hollywood actor in this besides Travolta.