I did a part-time Education master's while working as a full-time teacher. It was a decent amount of work, but not unmanageable. It really depends on the program though--my program was asynchronous and meant to be completed part-time while working as a teacher, and I completed it in three years. I would recommend learning more about different programs and inquiring as to how many students complete it while working full-time--maybe see if you can contact any current/former students. I imagine any program that can be completed part-time with night courses is meant to be completed while working full-time, but still it would be good to find out what the expectations are. If it's financially doable/allowed, extending it to a third year (if it's a 2yr program) may also be a good idea. Good luck!