In college, you should take psychology courses, preferably including some in adolescent development and in social psychology. I would also suggest getting experience with high school kids (for example, maybe in a tutoring role, or as a camp counselor).
There are some graduate schools that offer school psychology programs. For example, this is the description for the one at Tufts: http://gs.as.tufts.edu/academics/graduateprograms/schoolpsych.htm
You could certainly search for others.
If your high school has a school psychologist, it would also be a good idea to ask to meet with him/her to discuss his/her career path. I'm sure he/she would have lots of good advice!