What timeframe did they film?
Starting with the second season, the intro had footage for Seasons 2 and 3. Then Season 3's intro had footage of Season 4 (notably the Pipe Dreams episode where they do the group high five).
My point and question is when did they film this? Seems like they filmed it in a short time period. I guess the timing in filming for shows was different back then than it is today.
Plus the posters that would pop up in a bedroom or the principal's office (Teacher/Student Week) got me wondering when things were filmed.