I.G.
I think you are being exploited. If you work for 24 hours, you should be paid for them. Now, that said, it is not unusual to be paid for part of it at a different rate (on-call/stand-by) for example at night when everyone is asleep. But if you are required to be at your workplace for 24 hours you should ba paid for the entire time.
As for the CNA class, it will definitely help your career outlook if you plan on staying in the medical field. It is often a first step towards career advancement, too (for example if you are considering becoming a licensed nurse). If you take the class at an accredited school, you may qualify for financial aid.
Good luck!