My life really wouldn't be that different (I don't think) if I was given a chance to do it again. There are, however, a few small things I would tweak if given the chance.
To my 16-year-old self:
Don't quit piano lessons. All of your friends are going to give senior recitals and you're really going to regret not being up there with them. Plus, deep down you enjoy playing and will miss it.
Don't buy the purple dress for your junior prom. Trust me.
Don't buy the "silver" dress for your senior prom. Seriously, trust me.
Contrary to what you claim (in a childish rant aimed at your business teacher), you DO need to learn how to use a computer, type and run a 10-key keypad. Also, the Internet isn't going away. Learn what a search engine is.
Take chemistry in high school. Otherwise, you're going to waste an entire summer in college sitting in a smelly classroom with a broken air conditioner.
While you're at it, take Advanced Literature too.
Get a job you bum.
Red is not your color. Keep it out of your hair and off your lips.
Don't train for volleyball by jumping rope on cement. Duh, you should know better.
Seriously, the purple dress, don't do it.