Is it legal to pay workers per "session" they work and not hourly?

I was looking at job listings on Indeed and stumbled across a company hiring some programming instructors for schools and I was confused by their pay structure. It said in the listing that the pay was $43–$44 an hour but in the actual job description it says "Pay is based on per session, not per hour. For example: For a one-hour class you are expected to be onsite 15 min before the start of the program and 15 min after the program for clean-up. $44 is the rate for the entirety of the session."

It gives this example of a one hour class but it specifies that you must be available to work for 2 hours at least at two different times of day (lunch time and after school hours), and have a car and be able to drive to get to and from schools requiring your services. If you were to do both of these specified class sessions within the timeframes it gives (11am-1pm and 3pm-5pm) you wouldn't have a ton of time to do much between shifts and would essentially be on call for a decent chunk of the day while only earning $88.

More to the point of my question, I think I just was curious if it is actually legal for a company to hire you as a part timer and then only pay you in flat fees for the shift instead of an hourly rate. I'm more than familiar with gig work as an artist but this type of job being structured in such a way was surprising to me. I'm always curious about employment laws and I found conflicting information online about this.