Direct mail died with the dinosaurs. Real advertising today is by social media or getting your name out there in front of your potential clientel.
If you want to focus in on high school senior images, become active in sponsored events like listing a paid advertisement in high school yearbooks, sports leaflets or school paper. I started shooting sports for our local high schools and now whenever they need a photographer, they call me. I also support the school and athletics with annual donations.
The only print advertisement we do is placing inserts in local hotels and restaurant folders they give out to brides and grooms. This works the best in terms of return on investment!