To the middle school me, I would tell her "Never to let anyone take a picture of you on a bad hair day." The frightening thing is that given the grin on my face, I was pretty happy with my efforts. Must have been a dress up day at school.
But in general, I'm not sure there is much I would be able to tell previous me's that would have made any difference. I have have made my mistakes and learned from them. They are the mistakes of youth. The mistakes we all have made as we travel through this life. They the errors that we are warned against, but we are young, we know what we are doing, we don't need to listen. And really, would you want to change your path? Remember, it is the one that brought you to where you are today.
Still, there is one thing that I do wish the young me would be willing to listen to. Perhaps because of where I have ended up professionally, I do wish that someone would have talked to the high school me about becoming an engineer. I imagine I wouldn't have listened. It's even possible that someone did. And then I still would have had to make it through college chemistry and calculus, both of which were enough to convince me to not be a chemistry major*. Still, there is a part of me that is awfully curious about what I may have missed by sticking with the social sciences.
_________ * For the curious, I ended up with a degree in sociology.