Word of mouth is going to be your best friend. Every photographer around wants to do senior photos, and they get bombarded with info....so you need to be different.
I have senior reps from schools....I do a free session for them, and then they give out their personalized rep cards to their friends, and make $10 off each referal.
Facebook and Myspace are good networking tools to use with teens too.
Also...in a lot of areas, juniors already have their senior photographer set up....so get them during their junior year if you can! (like around Jan-April) A lot of them do senior pictures right as summer is ending or their senior year is beginning, so you gotta start early. Find out what is the norm in your area.