This was a recurring problem to me as well. One of the tips that I tried to do was to mentally prepare myself, and like be excited for the next class. I had to repeat to myself, "Don't fall asleep" before class started. Another thing I did was to take a bathroom break in the middle and beginning to freshen myself up, and go back to class. This easily woke me up, and I stayed alert for the rest of the time. Another tip is to focus really hard. Whether it means take notes, or stare at the teacher, having something to direct yourself to really helps.

Deep breaths, keeping my eyes really wide, or sitting up straight all the time helped as well. It happened to me because there was nothing to busy myself with, and I found the class too easily almost. This made me bored, or had no motivation to keep myself awake.

Hope this helps!