From February 1977:

He was the “kid from Kotter” and his “disco movie” had just begun filming and was called “Saturday Night.” There was also “Grease” talk. He also wanted to play more mature roles. Like someone in their mid-20s. 

Interestingly, right next to this profile, there's an ad for the movie “Rocky,” which had just been nominated for 10 Oscars, including best actor and screenwriter for its star Sylvester Stallone. Six years later, Stallone would take the kid from Kotter and put him in a sequel to the disco movie. He would train Travolta, get him sleek and glossy, and star him in a sleek, glossy and awful, awful movie that would bomb.