Naturopathic doctors call it the "Square Breath", and I use it all the time at school (and in my life). It's super easy and fast.
All you do is breathe in for 4 counts, hold for 4 counts, breathe out for 4, and hold for 4. Try that a couple times between classes, and I'll bet your stress will calm way down. It also helps you concentrate and get present with your kids after a moment of overwhelm or negativity.
Let me know if it helps!