1. Chewing gum/mints

It's the most obvious option, so it makes sense that it tops this list. Just don't make it obvious in class by being loud with it!

2. Drinking/snacking

Again, an obvious choice: both to get you up and stay hydrated at the same time. Just make sure your professor allows food and/or drinks (other than water) in the classroom or lecture hall. Regardless, even if it's just water, a sip or two should be enough, especially if chilled...

3. Fixing your posture

Have you ever noticed why we yawn? A simple question that isn't asked that often, despite how commonly this happens from time to time. Answer: when we slouch over, clearly tired, we narrow the breathing pathways in our lungs, and this is just our lungs telling us to fix the problem so they (we) can breathe properly. So, try sitting up straighter in class!

4. Having a neighbor keep you up

It helps to know at least one person in class and sitting next to them. Or it gives you the chance to make a new friend and ask them to do you a solid on an occasional (or a regular) basis.

5. Snapping a rubber band/hair tie (lightly and sparingly)

This I don't recommend too much due to the physical consequences as well as what it tells other people when they see this around your wrist. Please be careful that it's not too tight to cut off the blood flow in your arm and that you don't snap it too hard against your wrist to cause bleeding.

A hair tie wouldn't be as painful to your wrist or as alarming to others as a rubber band, while the latter implies signs of self-harm, so using both for this purpose should be very few and far in-between. If you have a rubber band with you and decide on this option, it's only to keep yourself from drifting and nothing else, and of course, make sure not to distract others while doing so.

6. Taking a walk outside

Just to go the bathroom, get some fresh air, or get some time alone to think. I've done this a few times when I kept dozing or couldn't concentrate on my work, and each time has helped me tremendously.

7. Surveying your surroundings

There's nothing wrong with looking around the room or hall, noticing anything interesting around you that may be of interest, something to focus on while you're holding on to reality if just the PowerPoint slides or the instructor's lecture aren't enough to grab you in (with no offense to the instructors, of course).

8. Writing a specific phrase over and over

I don't drift that often, but in rare cases that I do and am fresh out of the first few options, or if I'm just bored and don't know what to do with myself until the class ends, I try writing down various things on my mind, including something I must do (or in this case not do) throughout the class in which I'm trying to stay awake or keep busy: by writing the phrase I will NOT fall asleep. In fact, it was while I was doing this that I came up with the very idea to write this piece!

9. Taking notes/making lists

Speaking of writing, you could write other things, like keeping track of lists of everything you need to do or jotting down notes that you need to remember for later. Or there's always, of course, taking notes on the things being covered in class...

10. Doodling

This is the age-old way of using your notebook by not using it for notes. Personally, it's surprising how I never kept doodles in my notebook, despite how artistic I am in nature (mostly in writing).

11. Writing motivational quotes/phrases

These will help in terms of getting past your hurdle of trying not to sleep in the middle of class by reminding you to stay motivated, even when your dopamine levels keep telling you to give in to the temptation to sail off to Dreamland. And once you've resisted all of that, you go on to…

12. Giving yourself a reward to look forward to

This is a sure-fire way to achieve your goals. Once you tell yourself, "You're gonna get through this," and you do, celebrate by treating yourself to something you enjoy (maybe a snack from the cafeteria or a dessert) for a job well(ish) done for getting through this class (hopefully not more) without sleeping! Just make sure that treat doesn't make you even more tired. I would suggest a nap as the most ideal treat for an accomplishment like this, but even that might turn against you.