Where do the 4 missing minutes go?

Why do you think it needs to add to 60? Those "4 missing minutes" are probably the first four minutes of the next period.

Many US high schools don't rely on hourly scheduling, so classes may end at arbitrary times: I finished at 2:11pm daily.

I thought it was just our school district. We got out at 14:32. Why not 14:30??

Classes are scheduled by hours / semester, not hours / day.

Adjust for how many days you can fit in around the major holidays, remodeling, and so on, then divide the number of days by the number of minutes and you have the required course time. (Of course you tweak it a bit if the course session lengths are odd.)