This is very true. The amount of time it takes to perfect and perform at that level is really incredible. The advanced show choir at any school will likely only accept performers that have already had years of training (similar to HS sports these days). Students begin in the summer with "camps" that are intensive combination of conditioning, dancing, vocal training and learning the first parts of the show. Because there is no physical contact like football, there are no rules about how long any of these rehearsals or practices can be. Students are generally required to not only attend all practices and performances, but they must also attend dance classes outside of school hours, often at a specified dance studio and they may also be "encouraged" to take vocal training outside. Since most of the students in those top choirs came with prior experience, they are often still enrolled at their "home" private studio where they also train and perform, but the HS choir demands top priority when there are conflicts. These choirs are a class period, plus after school and since they don't count as a sport, they take the place of an elective. Sports teams take the place of the PE class. In addition to the class period, there are at least 2 days a week rehearsals that intensify during competition season.
I had kids in show choir and it was not at all uncommon for them to have something going on right after school and then rehearsal until late and then they could start homework (often no earlier than 9pm). Competitions were almost always on Saturday. When they travel to something like FAME in Chicago, that's a week off of school.
Being that none of it is regulated by anyone and most teachers don't much care how much rehearsal you had the night before (not sure they should care), students feel pressured from both sides. I've seen it cause breakdowns from the stress.
Thousands of tired, nerve-shaken, over-civilized people are beginning to find out that going to the mountain is going home; that wildness is necessity; John Muir