Try to think of Head of the Class as the flip side of Welcome Back, Kotter.
Whereas in the latter series, teacher Gabe Kotter (Gabe Kaplan) had returned to his high school alma mater to teach the same remedial education course he'd taken as a student, aspiring actor Charlie Moore (Howard Hesseman, ex-WKRP in Cincinnati) lands a job as a substitute teacher for a class of academically gifted teenagers. Six episodes in, we learn that the teacher Moore is filling in for (Roscoe Lee Browne) isn't returning, so Moore stays on as a permanent teacher. Well, at least for four years. Hesseman left the show after four years, and Scottish comic Billy Connolly stepped in as a completely new character in the final season.
Class introduced viewers to two future producers in Brian Robbins and Dan Schneider, both of whom cut their teeth behind the cameras at Nickelodeon (Schneider's still there), as well as future tabloid bait Robin Givens, whose ill-fated union with boxer Mike Tyson, for all intents & purposes, derailed her career, and second generation comedienne Rain Pryor, who's currently doing a stand up tour in tribute to her father, Richard Pryor. Dan Frischman (Arvid) would find his way to Nickelodeon as well, as Robbins cast him on the All That spin-off, Kenan & Kel, as a foil for Kel Mitchell & Kenan Thompson.
In 1988, Class became the first American sitcom to film an episode in the Soviet Union, breaking new ground at a time when the Cold War was nearing its end.
Here's the intro for the first season:
I think part of the reason Saved by the Bell's primetime spin-off, The College Years, flopped in 1993 was not just because of fatigue with original cast members, but the fact that with those characters three months removed from graduating high school, viewers were expecting an upgrade of sorts from both the parent Bell, as well as Class, which preceded Bell to the air by a couple of years.