It is better if you take art classes in high school you will get a better feel for what it's like to be in college art classes. Plus it is important to be consistant with your art making rather then sporadic. If your lucky you will have a good art teacher plus if your an artist (in high school) art class may be one of the few places you look forward to going to.
No, although it is good to start, so you know that's what you'd like to do. Better safe than sorry. Depends on what College you'd like to attend. Some Art Colleges might want portifilloes before they accept you.