There were 4 main students that the show focused on, collectively known as the "Sweathogs". Each had a distinctive personality with a tagline. Unlike many other shows' taglines, I usually found them to be pretty funny.
Arnold Horshack was the white kid who yelled out "OOO!! OO!! OOO!!" with his hand up when he knew the answer to a question. He also flattered the teacher with "Very impressive, Mr. Kotter" from time to time.
Freddie Washington was the suave black basketball player with the pick up line "Hi there". Typing it doesn't do it justice...those who watched the show know how it went.
Juan Epstein missed alot of class but always had an excuse note that he would write, supposedly from his mother, which was signed "Epstein's mother" each time.
Vinnie Barbarino, played by Travolta, was the "cool" one with the phrase "Up your nose with a rubber hose".
Yeah, it's a bit dated now....and Kotter's jokes to his wife and class were always pretty bad. I think the students made up for that though and usually made for an entertaining episode.