Quite a few things actually.
I would have been more forceful and vocal in defending my close friends when they were bullied.
Those dudes never tried it with me for two reasons. First because I was definitely in the top 5 biggest/strongest kids in my grade, also as awkward as I was, I think my place in the social ladder was just above "bully-me at-will"-status.
Unfortunately, two of my closest friends weren't quite there, and they used to get clowned on a daily basis. At the time, I just shrugged it off as 'harmless roasting' but looking back, the frequency with which it occured, it was definitely bullying and I should have stood up for them
Would have grabbed my crush's ass right back, after she slapped mine, instead of just standing there with the Goofy "Oh Shucks" expression.

@ how I didn't even
try to holla at her. She had made so many moves on me and was making it so obvious at that point
Also, (with the benefit of hindsight) I wouldn't have let my place on the social ladder have affected me as much as it did.
Even though I wasn't nerd-status, and was friends with quite a few of the cool kids, I was never considered to be one of them, if I went back now I wouldn't have let that shyt bother me.
I would have just talked to the people I was good with and wouldn't alter my behaviour for anybody. Basically, I would have been
myself 100% of the time, and not just in front of my close friends. My attitude would have been, "This is me, ya'll. You either fukks with me or you don't

"
Another one, instead of admiring the most popular girls in school (that I would never get

anyway), I would have gone for the girls, outside the popular group who
I thought were cute and wouldn't give a fukk about their place on the social ladder or what other people thought about them.
On the sports side, I would have done A LOT more cardio, to make myself the fittest member of the team. Even if my actual skill level remained the same, an improved level of fitness would have given me the confidence to take more risks and to maintain concentration near the end of the game when others were losing theirs due to the fatigue