Pumping at Work

I'm a middle school teacher and just got back from maternity leave. There is one point in the day where I don't have a scheduled break so someone comes to my room to watch my class for 15 mins so I can go pump. My students have seen me leaving with a black bag in my hands for a few days now and I have heard them whispering "where is she going?". I know it's only a matter of time until one of them directly asks me where I go every day (or asks the sub who watches them). 

I don't want to lie but I don't want to tell them I'm going to pump breastmilk. 

Any suggestions on how to phrase it so it sounds like a teacher/work responsibility and something that they won't question and ask more about?