I am a teacher (kindergarten) and I went back when ds was 10 weeks old.
It worked for me to just pump at lunchtime. For those early months I had a good enough supply to be able to pump enough at that one session (with a double electric pump) to leave dh with enough to feed him while I was gone. When he got older and was eating more, I added a pumping session at home right before I went to bed. I also had a freezer stash (from those early days of oversupply) that we used quite a bit of later on.
My principal was super-cool about it because she was a nursing mama who pumped when she was a teacher. It also helped to have teachers around me who covered my classes on those rare times that I had to pump when my class was around. For instance, on field trip days! I had to wait until we got back to school and then have someone cover my class.
For the most part I pumped in my classroom while my students were in the lunchroom. Sometimes at my school the students have to eat lunch in the classroom. For those times I borrowed a tutor's office. She was very accommodating and gave me a key.
If you need to pump at other times, surely you will find a way to work it out. Your time at school is precious, so it was an adjustment for me to take time out to pump. I am usually the type of teacher that does not take the time to sit down for lunch. Rather I'm rushing about setting up centers, making copies, marking papers, dealing with discipline issues, etc. But I loved having that time each day, even if it was just 10 minutes, to set work aside and do something just for ds.
One thing that helped was having a mini-fridge in my classroom. I also streamlined my pumping process. For instance, I never rinsed my pump parts at school. After pumping I poured the milk into the storage bottle, stuck that in the fridge, and put the pump parts back in the carry case. I took care of washing at home each evening. I also left my pump set up and plugged in at school so it was just a matter of attaching the horns each day and flipping the switch.