1. My team meets mon-saturday regularly through the summer, with our coach showing up at least on tempo days and long run days (so 3 times a week), and randomly on some other days. We run at a park near our campus at 6:45 and finish usually by 8:30. Rides can be an issue, but upperclassmen are big helps, and it also builds a team bond. Often parents drop off their students before practice, then a couple upperclassmen drive everyone home.
2. My coach makes a spreadsheet I believe, with each athlete's name on the left, and squares for each day over the summer. After our run he has us pencil in our mileage. It's not enough to show everything everyone has run over the summer, but it gives a good idea of their miles per week. We also got t-shirts for 250 miles, 400 miles, and 550 miles.
If you want highschool athletes to run during the summer, you're going to have to push most of them. Creating a regular, routine meeting is very effective. Keep encouraging them and working out your plan for summer before school lets out, and find your highly motivated guys and use them to help motivate the rest of the team.