Is the actor that played newman on seinfeld the same guy that played the principal in billy madison?

No. In the 1995 Adam Sandler movie Billy Madison, Principal Max Anderson was played by Josh Mostel while Wayne Knight is the one who played Newman in the TV Series Seinfeld.

See full cast of Seinfeld and Billy Madison on IMDb.

Actor Wayne Knight achieved television immortality assaying the role of the frenetic mailman "Newman", nemesis of the eponymous lead character in the classic TV series Seinfeld (1989). He also appeared in Jurassic Park (1993) as "Dennis Nedry" and in one of the most notorious scenes in American cinema as one of the detectives interrogating Sharon Stone during her leg crossing in Basic Instinct (1992).

A portly character player of TV, film and stage, Josh Mostel may be best remembered as the uncle who eats pork on Yom Kippur and gets sick in Woody Allen's nostalgic "Radio Days" (1987) and as the ice cream magnate with a knack for choosing just the right flavor for any occasion in Ron Underwood's "City Slickers" (1991) and its sequel.

View Wayne Knight and Josh Mostel filmographies on IMDb.