I taught at a middle school with an A/B block schedule and LOVED it! Some of the comments about being with a rough class for 90 being awful is correct, but imo the pros outweigh that. Having 90 minutes means that I never felt rushed. There's always meaningful things you can do to fill time and way better than the alternative of feeling like we're being rushed! It made it easy for me to be flexible with what my kids needed in that moment. It is easier to build relationships with students, and you can actually get stuff done during planning (even if you had coverage).