DR Jennifer - The class is "voluntary" - more or less. However, it is a great warm-up, and it's a true dancer's warm up - concentrates on stretching, flexibility, balance, line, strength, etc. However, if someone has their own personal warm-up, or needs to warm-up differently due to injuries or something like that, then they can do the parts of the class that work for them, then do their own thing when it suits them.
I do a "modified" version of the class, mainly since I can not touch my toes, nor bring my forehead down to my knee caps. But I do what I can. My crunches and sit-ups may not go as "far" as the other people in the class, but I do pretty well - especially since I'm just "the pianist".

-Oh, and after only two days, I can already tell that my jeans are fitting better. -And the "exhaustion" has also been helping me sleep better too.