From Muppet Wiki

Dr. Hugo Krassman is a science teacher at George Washington High School and serves as the antagonist to Kermit and his friends in Kermit's Swamp Years. Krassman is devoted to gathering frogs for his biology class, and browbeats his assistant, Mary. He also takes great offense when people address him as "Mr. Krassman." His antagonism towards amphibians stems from a childhood trauma.

Dr. Krassman was named as an allusion to Professor Krassman from The Muppet Movie. In the DVD audio commentary for the film, Horace D'Fly asks whether the two are related. Kermit replies that "maybe we'll find out someday."