If there's no special needs or food sensitivities, there's really not one that's better then others. They basically have the same nutritional content from the same ingredients. You just don't want to be switching every time a new coupon comes in the mail. The advice I got (from Lara Zibners, author of If Your Kid Eats This Book, It Will Still Be Okay) is that store brand is fine.
Your principal needs to get a floating sub in there for you so you can pump more frequently.
A quick search found this: http://breastfeedinglaw.com/state-laws/oklahoma/
I hope that's accurate...if it is, you are covered. I spend oodles of time at my son's school and I know there are floaters on campus all day long who relieve teachers who need to duck out for a quick pee break. The front office clerks also cover classes for 10-15 minutes at a time. So does the dean when no one else is available to cover. Sometimes, fingerprinted parents who are doing lunch duty or running the book fair step up to help out. It may take some effort, but you should be able to make arrangements for more pumping so you don't have to resort to ABM.
In case you need to go with formula:
Avoid formulas with palm oil, I've found two separate studies showing that it inhibits fat & calcium uptake and leaves residue in the intestines that causes harder stools (after switching to a formula without, I can't help but think this is part of the reason so many formula fed babies get constipated).
Read this about organic infant formulas: http://foodbabe.com/2013/05/28/how-to-find-the-safest-organic-infant-formula/
I didn't go organic, because I couldn't find a decent one. As others suggested- I'm only willing to get a formula with lactose rather than corn syrup, sucrose (Similac Organic, I'm looking at you), brown rice syrup (BE CAREFUL WITH THIS, these formulas have unsafe levels of arsenic unless the company specifically gets rid of it somehow, the companies are- but I know people who recently bought formula and found, based on expiration dates, it was from before the recall), etc. I'm also admantly refusing to get anything with palm oil after the months of pure hell that was formula with palm oil.
The organic formulas without palm oil don't have lactose. The organic formulas in the US also don't seem to have lactose-only, including corn syrup/maltodextrin as well. Some used to, but then switched to something else- very frustrating.
Liquid, ready-to-feed formulas often contain carageenan, which causes inflammation. (diet drinks, breakfast shakes, etc also contain it) The way they put DHA/ARA into formula is usually bad, but I've yet to find a formula in the US without it.
Check out Human Milk 4 Human Babies in your area if you're comfortable with milk-sharing.
Similac advanced. They also have a Supplementation- I haven't done a complete comparison, but they have the same basic ingredients. If you can afford it, a lot of people import formula from Europe- that's something to look into. I think you can get them on amazon for not that much more than US formulas.