It's difficult, but you may not have to wean during the day. I think it's much better to pump during the day than try to pump a bunch now and save it. You want your milk supply to stay "up" and you want her to get the milk that is most appropriate for her, as it changes with time to meet her needs. Milk pumped now isn't "perfect" when she's 6 months old, though it's still good.
I was a teacher, and felt like my lunch break wasn't enough time for me to pump. the ideal times for me were 9:30 and then during my conference period at 1. It only took 20 minutes from my conference period, so I could still do other things (unless we were having a meeting, and then I pumped during lunch and immediately after school). I also arranged with the principal and another teacher to combine our classes for half an hour in the morning while she read a book and I pumped. Then I would take over and do the rest of the lesson with the class. It was perfect, and I constantly thanked my friend for helping me.
At those times, I pumped in my classroom at my desk with the door locked. You probably won't have that luxury.
Other teachers I know would use the bathroom. I wouldn't do that unless I HAD to. We had a workroom that was storage for the grade level stuff. It had a sink and outlets. I used that several times when my students were in my classroom. A lot of other teachers just pumped at lunch and at conference times, but those times weren't spaced out enough for me (my conference time was only about half an hour after my lunch time).
Talk to your mentor teacher and your student teaching facilitator and see if they have any suggestions for what people have done in the past. They should be willing to work with you. You shouldn't have to wean and give formula during the day if you don't want to. I pumped and nursed, and my son was exclusively on breastmilk for the first 7 months, slowly started solids and still mostly drank breastmilk for the next few. He never had one drop of formula, and I worked full time with children. It can be done, but it will take some help from the teachers around you to find the way to do it best.
YOu may not be pumping much now, but when you're away from her, you will get lots more. As you are pumping what she should be eating, you'll see. Get a few days' worth before you start working, and then what you pump one day she'll eat the next. It's always nice to get some excess, but don't worry about pumping everything now. A good double electric (Medela PIS, for example) is going to be best for you. If you find you need a little help pumping, try drinking Mother's Milk Tea three times a day. Eat oatmeal in the mornings!
As of yet, there is no Federal law that protects breastfeeding mothers. The laws on breastfeeding and pumping differ by state, but in Texas, they don't legally have to let you pump at the workplace. THere is no law in place to protect us for that. However, Texas law states that a breastfeeding woman has the right to feed in any place she's legally allowed to be. It also allows workplaces to promote themselves as "mother friendly" if they comply with a list of provisions. http://caselaw.lp.findlaw.com/txcodes/he016500.html But the law doesn't say a workplace HAS to let you pump. I took it as, "I can feed my son here, so unless you want me nursing my son every few hours, let me pump." My mother said she'd bring the baby up to the school if they wouldn't let me pump! It never came to that, though, because I was very sweet in asking everyone around me for cooperation.