Don't let companies/people brainwash you into thinking Unions are bad. As someone who has worked for a union and none union company, I can tell you union companies treat you so much better, I went from having a forced clocked out lunch to a paid lunch, anything over 8 hours a day is time and half and so many more benefits (I won't bore you with them). Plus the union place functioned miles better than the none union place.
Unions offer you protection and a lot of benefits. But I would suggest you talk one on one with your union representative at the school to learn more about your union. If this is only a temporary job, I'd avoid joining the union. But if you plan to stay long term, your better off joining the union. Also, check if you are in a 'right to work' state, you might be forced to join if your not.
Again, Unions in the private sectors do wonders for there employees and keep corporations honest. I made more money due to being in a union position. Most union horror stories you hear are government unions that get there employees great package deals, but even this is way over dramatized by the media.