You can go to the nurse. If you don't want to be missing classes then you should buy Thermacare heat wraps from the grocery store. They're heated with a sticky side so you can wear it under your shirt and no one can tell. However, make sure you don't wear it too long. It's only to use for a few hours so probably during school would be fine. You could also take pain relievers such as Tylenol and Advil in the recommended amounts
train yourself to have a bowl movement every morning before school. pick a time and try every day at that same time. before long your body will adjust, your movement will be successful and no more cramps.