This was possible while I was in high school -- I was able to take Bio, Chem, Physics, and Calculus in the same year.

However, this is no longer possible now for my younger siblings enrolling at the same public high school. Apparently the school now put much more strict limit on pre-requisite and what classes students can take at which year. There is now much less flexibility.

This is not due to significantly increased student body size , but mainly for "equity" in education outcomes -- i.e. holding back the advanced students to accommodate the less advanced students. It makes very little sense, but that is California!