They are expected to work about 20 hours per week during the school year. That amounts to about $33 per hour on a PART TIME JOB (those hours are supposed to include planning and grading). FYI $33/hour is a living wage if you work full time.
When I attended graduate school, I was a FULL TIME high school teacher and took out student loans. It was worth in the long run as I got a raise for my masters degree and another raise when I hit master's Plus 15/30/45 hours.
They are getting paid far more per hour than full time starting school teachers who must actually have teaching credentials in pedagogy, instruction, and the content area that they are teaching and are in front of a class of students for more hours per week (approx 40) and must grade, prepare, communicate with parents, complete mandatory PD, etc. on their own time.
I have seen far too many students GPA and self esteem ruined by graduate assistants masquerading as teachers in front of megaclasses of hundreds of first year students.
If they want better, get better qualifications for the job that you are theoretically hired to do. Far too many graduate students think that they are getting paid to work on their thesis or dissertation and not actually teach the undergraduate students thrust in front of them.
I am pro union (was union rep for my school district for years) -- but when it comes to being paid to teach, you better actually put student learning first and be qualified to do the job.
I couldn't hire the people hired as teaching assistants in colleges to a long term sub job -- because they aren't qualified for a TEACHING job. If you want to pay them to be a lab assistant, then give them that title an pay them entry level wages for QA technicians.