Hi A.!
First of all, I applaud you for your dedication to breastfeeding your girls while going back to work. I did the same thing all last year, so I know the challenges you are facing.
As I scrolled down through the responses, I see you've received good advice. Here are some additional comments I thought of for your situation:
Make sure you have a good quality electric pump. It is worth the extra money, especially when you'll have limited time to pump. I also found that a second set of shields & tubing was helpful. One set was packed for the next day while the other set went into the dishwasher to be washed/sterilized.
Find a place that is as comfortable as possible for you. I know that may be difficult in a school (I had to pump in the handicap rstroom next to my classroom), but bring in things that will make it work, if possible -- a table, comfortable chair, maybe a foot rest and a blanket.
Be sure to have some water and a nutritious snack when you pump. You'll be amazed at how much energy you put out when teaching, and your body will need the extra nutrition.
Get LOTS of rest. What I remember the most from last year is how exhausted I was each day -- mentally exhausted after school and physically exhausted by bedtime. Try to cut corners at school as much as possible. Delegate any tasks that can be delegated, and take home the least amount of work that you possibly can. (Better yet, don't take anything home!) You will want to spend every precious moment in the evenings with your girls. I also found that during the times when I was under stress at school (aka report card time), my milk supply dropped to it's lowest. Stress and lack of sleep can affect your milk supply, so take it easy during the busy times of the year.
Most of all, just try to relax. Within a week or so, you'll get into a routine that's comfortable for you. And if, for any reason, you have a difficult time keeping up with your daughters' supply needs (as I did), don't be hard on yourself for having to suppliment with formula. Any amount of breatmilk you give your girls will help them.