They are 20-30 year old grown adults who went back to school to get their GEDs. They are in Adult high school (like my mom went to briefly in the mid 1980s).
These are not 9-12 grade high school students and they are not 'teens'. Courtney Thorne Smith's character was a 19 year old who went back to school (i.e adult high school) to get her GED so she could go to University . Her character dropped out of high school at 16 (in about 1983/1984)
THis is canon and true

I think they were in either 10th or 11th grade. To me they seemed more like 11th graders, but I do recall Pam saying that she was 16. Commonly, Sophmores turn 16 during their year, and Juniors turn 17.
However, Pam could have been born in the summer which would make her birthday come later than everyone else in the class.
I dont recall a grade being officially mentioned in the movie, so we'll have to speculate.