The only things that would be worth changing:
1. Actually get some of the contact info from some of my old friends instead of just letting them fall to the wayside after graduation.
2. I would have taken my health more serious because you can't do the same shit when you are older. I just had to get over $2k in dental work because I didn't care to go to a dentist for five years and drank soda everyday. I would start eating right instead of eating whatever processed, sugary crap was around. A good few years of back problems and joint problems could have been saved if I paced myself better and took the proper precautions.
3. I would take college courses instead of the ones at the high schoo. I would have CLEPed out instead of doing AP. If I went that route, I would have a degree by now instead of having to do another 2 years.
4. I would start assembling my portfolio then instead of waiting for college. By portfolio I mean all the theatre sets I built just to show I knew wtf I was doing back then and still know more than half the dolts here.
There are always the large things too where people have died or been murdered, but I think experiencing that in high school better prepared me for life as an adult. So, as much as I would like to have kept those people in my life, what happened was too important to really contemplate changing unless I knew the consequences of those actions.