I think that part of the problem is the difference between getting people to join the site and getting people to join the community. The first merely requires that a user set up an account, whilst the latter includes (but does not require all of) reading through (and possibly answering) questions posted on a regular basis, taking part in the review process and participating in or reading meta. High school students simply do not have the knowledge to read through most of the questions on this site and gain something from it, let alone attempt to answer them. They are therefore not able to really "join" the community, but are only really able to use this site as a means of occasionally asking questions.
I would contend that a fair number of people on this site are in high school. There are many elementary calculus questions on the site, as well as a great number of introductory probability-type questions (including permutations and combinations, among other things) that could all reasonably be asked by high-school students. I think it is simply that this site offers less for people who know less maths.
(I feel that I should point out that I am not saying this in a derogatory manner, it is certainly possible that some high school students may gain a lot from the site. Speaking as an undergraduate who doesn't understand the material covered in a great many of the questions posted, and who would certainly have understood a lot less when I was in secondary school, I know that I would not have gained much from this site before I had done a reasonable amount of university-level maths.)
As to how we can fix it, I'm not sure. It's been my experience that most high-school level questions don't have that much depth to them. Thus, if someone is struggling with a particular question they are likely struggling with an entire concept. This means that they are unlikely to be able to provide much information on what they have tried already, and the asking of low-level questions without providing this is often frowned upon.