And yeah, Orbito, practice is the only thing that matters. You're in highschool, so I'm sure you have at least one class where you know most of what's being said already, and my suggestion is...doodle. Doodle doodle doodle doodle doodle. Doodling, but doodle how you usually draw. The more doodling you do, the more practice you get, the better you get. So, doodle. Get a little notebook, half the size of a normal notebook, and dedicated it to doodling. NEVER doodle in school notebooks or school books, but have that little doodle notebook sitting in a little corner of your desk. I actually used my assignment notebook this year, because it's just the right size and has tons of extra space besides where I jot down my homework.
Oh, and if anyone ever tells you to not do it in their class, stop doing it in their class.