Don't try using MySpace to locate the seniors, you'll just frustrate yourself. Go to the school itself.
Make some flyers or brochures, and take them to the school.
The school probably won't let you pass out unsolicited materials, but you can drop them off at the front office, or ask if there is a bulletin board you can post them to.
One you have one hooked in, you can use that one student as your 'in.'
Give that kid kick backs for every student that he/she brings your way.
 

There are many ways to find seniors, but nothing else will come anywhere close to the results you'll get with a Yellow Pages ad that says "Senior Portraits."
 

Find yourself a guy or girl that's around the age of a senior that you can call a senior, offer them a free session they have to buy the disk or prints they way... use them to make an ad, and make it spiffy something that will catch the eye of a teenager and a teenager's parent (hard I know but you can always make two flyers) leave them up at your local panera, concert places, anywhere a teen will frequent...

Otherwise there are services that will give you names and addresses for a hefty fee then you would still need to mail mailers for the teens...

That's just my two cents...
 

Starbucks is a local hangout at 3:00 PM for teens.
Also church is the BEST suggestion.
Show up on Youth Group night...only if it is your church with flyers.
(don't go if it is not your church....not a good idea!)
 

I asked my friends if they know any seniors who as it turns out they do even if its the neighbors sons girlfriend. This is my first year taking on the possibility of shooting senior sessions so i started asking everyone I know. There are lots of high schools in my town so I have started an ambassador program offering a free session to one senior at each school who in exchange can earn free prints and $5 for each senior they refer and books a session with me.

Also don't forget to bring it up to strangers you do business with. I just got my next lead two days ago because i brought it up in conversation with the guy working on my car. it turns out he has a senior who is VP and very popular.

Have you tried Craig's List. You never know who might be looking online.