I do not regret anything I did in high school or college -- most of my regrets focus on the things I didn't do for one reason or another. So, my advice to everyone in high school and college is to:
1.) Use this time to figure out what you think you want to do -- really, what YOU want to do with your life. If you think going to school every day for 12-18 years is bad, think about picking a career that you don't like and doing that for the REST OF YOUR LIFE!
2.) I wanted to be a teacher -- I REGRET not really thinking about the lifestyle I wanted to have and realizing sooner rather than later that I wasn't going to achieve that life on a teachers salary. I had to choose a new career path in my late 20's after I'd paid for college 2x due to this late realization. So, consider both your passion and the funding you need to have a happy existence (don't think rich, think comfortable), as well
3.) Travel -- I grew up very poor so travel wasn't something I had the opportunity to do too much of. I'd never taken a train, plane, subway, public bus (thanks to growing up in the country), and I REGRET turning down opportunities to travel to places because I needed to know how to do these things and wouldn't ask for help. Traveling and seeing your world from the perspective of someone in another city/state and especially country can be a game changer if you let it.
4.) Experiment -- Not with anything illegal, but wear that outlandish outfit, get a hip hairdo, hang with a group of people (again, not druggies or thugs) that you might not normally hang with (I was a jock-- in college I hung with the Theatre kids... eye opening experience), try Hummus, etc.. I REGRET not experimenting more!
5.) Be you! Being an adult comes with lots of requirements to conform to be accepted - not much different than high school, really, but not conforming can cost you jobs, promotions, etc.. I REGRET not being more in touch with who I really was during that time and just being OK with being that person for a while. I missed my first opportunity to get comfortable in my own skin that first time around.
Laugh, love, have fun. Good luck!