In the pilot episode of Beverly Hills 90210 Brenda tells her (older) date that she is a junior in high school. David and Scott are freshman.
At the beginning of season 2 it’s summer in Beverly Hills and when school starts again, the kids are in their junior year.
The start of season 3 is once again the summer, and when school starts the kids have become seniors. David, however, does a bunch of extra work in order to graduate with his friends, seemingly only one year early.