First, good for you for looking for ways to supplement your income while in college! There are lots of certification programs out there, but as others have mentioned, some (most?) are not very well-respected, and those that are (PATH) are a huge commitment of time, so while very worthwhile, maybe not the best fit for you if your current goal is extra income.
Considering the insurance issues and lack of a good lesson horse, I’m going to throw out some advice you didn’t ask for. Hope you’ll forgive that and believe I’m doing it with good intention.
Rather than teaching riding, I suggest getting a straight-up part-time job to supplement your riding expenses. I know, that doesn’t sound like nearly as much fun, and it probably won’t be. But–what are you planning to study? If you could find part-time work that was relevant to your chosen field of study, it would
A: supplement your income;
B: get you relevant career experience that would will help you land a job after college; and
C: help you find out early on if what you think you want to do for a living is really what you expect it to be.
Then you can continue to ride/enjoy your horse on your own terms. Of course, depending on what you plan to study, it may be hard to find part-time work doing something related, and the hours would probably be less flexible, but I think if you could pull it off it would serve you better in the long run, without putting you at risk for a liability lawsuit.
Just food for thought. Good luck at college; it will be an amazing experience!