Is there a way to add a donation field in the checkout process that allows classmates to throw in some extra money while they are paying for tickets?

You can't really add it during the checkout process, but you do have two options:

1) In your list of items for sale, include one for donations and set a fixed donation amount. Your Classmate can choose to donate, or leave it inactivate and not donate.

2) You can put a separate "Donate Now" button anywhere on your site that Classmates can use to donate any time, either inside or outside of your Reunion Planner. Instructions for doing this are in this thread.

We've added an item called:
Optional Donation for Financial Support
and classmates can choose to donate at checkout. This has been very successful for us.