You didn't mention where you live, but I know that, sadly, many school systems around the country have eliminated or scaled back on PE programs due to budget issues. I say sadly because childhood obesity is on the rise and physical activity correlates positively with academic success.
The best way I know to get a child this age to exercise with some regularity is to plan for physical activity to happen as a family event. Part of the routine. After dinner, everyone takes a walk together or plays catch of does yoga. On Saturday, perhaps there's a family hike or bike ride. You can start small, a few times a week, and grow it. These are not activities that require spending money, just spending time. Take advantage of summer to establish new routines.
Limiting screen time is also important. Use a timer. It eliminates the need for you to be the bad guy.
Whatever you do, don't make exercise punishment.
Coincidentally, there was something in the Globe this week about a mom in the Boston area who was also unhappy at the lack of PE. She began a before-school exercise program called BOKS (Build Our Kids' Success). Reebok International has become a partner and the program is now in 25 schools in the Greater Boston area. Get in touch, it sounds like they are anxious to spread around the country! Nothing like some peer pressure to get a kid moving!