Seeing as how you know the families, you could call, send a postcard, letter or flyer. Whatever it is, it should contain a link to a page that contains an opt-in form, along with some bullet points about why they should opt-in. If you can, include some kind of incentive for them to opt-in, such as a free report. If you are a coach, there are a lot of things you could use for a topic for a little report or a 5-lesson email series.
Advertising your athletic development business outside of this group is beyond the scope of this reply, but you have a bazillion options.
Obviously, the opt-in form is hooked to an autoresponder service. I use AWeber. And even though everyone knows you, I still would use a double opt-in to protect your butt.
Last, since you are dealing with high school kids, I would consider also building an opt-in mobile phone list. Some providers of this kind of service have an option to not only send out text messages but emails as well to cell phones. Text messages get forwarded all over the place and this could lead to other high school athletes in other schools.