Throughout the series there are shots of Miss Li talking to the students in the auditorium and Trent (Jane's brother) is clearly visibly in many of the shots although he's much older and had already graduated high school 5 years earlier.
When Tiffany Blum-Deckler is introduced in "The Invitation", she talks in a valley girl voice. In all subsequent episodes, she speaks in a monotone voice.
At Lawndale High, the boards in the classroom alternate from being chalkboards and markerboards.
In "It Happened One Nut" Kevin walks into the back of the nut stand in a coat and tie and in the next shot he's back in his usual football uniform.
Tiffany Blum-Deckler has a Valley girl voice and appears to be somewhat intelligent in the first season but by the second season and remainder of the series she now has a more monotone airhead voice.
The Miss Barch character would not pass today, not because of her extreme feminist views, but because her character advocates domestic violence towards men. Throughout the series there are clips of her assaulting male characters on the show. She also has a strong disdain for Kevin and Upchuck. That wouldn't pass in real life if a teacher threatened to perform violent acts on students.