I'm hesitant to answer this question because after high school I became friends with some amazing people who helped me change into a much better person and were there when I needed them the most. I couldn't ask for better friends, and if I decide to go back to when I was 16, I'll have to find some way to become friends with them again after high school. This would be difficult because I became friends with two of them due partly to my idiotic faux-depression when I was 17-18, and one of them introduced me to a friend of her's. I practically owe my life to these three people, after everything they've done. Therefore, if I go back to when I was 16, I stand to lose A LOT.
That being said, if I do go back to when I was 16, I'd actually do my work in high school and get NCEA Level 3 (I currently have NCEA Levels 1 and 2). I'd also stop acting like such a weird, dodgy person, and this time around I'd see right through those girls' attempts to trick me into thinking one of them wanted me to ask her out. After high school, I'd have to find a way to become friends once again with K + C + H.
Ah, I just thought of another drawback to going back to 16 - when I was a teenager, I was really controlled by my dad. I had very little freedom to do whatever I wanted to do for fun. I wasn't interested in drinking or going to parties or smoking or doing drugs or even having sex, but up until I was 19, I wasn't allowed to stay up past 10:30pm and wasn't even allowed to have my computer on for any reason whatsoever (not even for playing music) unless I asked him first. And even then, he'd usually say no. Hell, he even controlled what music I listened to. All the little things I couldn't do, all the ridiculously insignificant reasons for which I was always punished... and all the rage that built up inside me because of his extremely heavy-handed methods of punishment...
Perhaps it's not worth going back in time, then.