<human>: Can you provide a brief rundown of the text in within 3 sentences?

I'm only a sophomore in high school, really organized, and easily interested. However a cancel of that is i'm also ADD. I'm constantly missing assignments and failing tests. Before tests, I understand everything. I could explain it to a 6 year old if i wanted to. The day of the test, my mind just goes utterly blank and i'm forced into using whats there on the paper. As for missing assignments, The only time i ever get to do them is taken up by yet another assignment. It's ruining my grades, frustrating me and I'm afraid it's gonna end up ruining my social life.  

What is something I can do to understand everything and get it done right the first time? I know, simply serious-the-fuck-up and get it done. but it's not that easy for me. Everyone i know is passing grades and keeping an awesome schedule, i'm busting my ass to keep a 80 or sometimes a 70 in most of my classes with no time to spare.
<bot>: I'm having trouble in school while all my friends are passing with ease, and I don't want the rest of my life to be this difficult. How do I stay on top of things?