I think everything happens for a reason, but if there was one area I wish I could re-do was my High School experiences. I was very shy and anti-social in High School, fortunately I came out of my shell in College. However, I've always wondered what it would have been like to talk to more people, do more things, and try new things back then.
I also didn't care much about grades and studying (which also was complete opposite once I made it to University). I think the early experiences in our life, especially High School, are important and stick with us. It's also one of the first times you're consistently working with others, collaborating, and learning foundational subjects that dictate your future interests and academic/career goals.