Any way you could use your school gym?
When I was in highschool we had weightlifting monday and weds after school. Maybe your school has some sort of thing like that, or maybe if you have a cool teahcer, you could convince them to supervise.
Other than that, you could probably find a decent weight set at a second hand store or something. When I worked out at home, I could usually find OK second hand stuff for my home gym home.
Also, pushups can be decent for building size in the early stages. Chinups too, you could get one of those chinup bars to put in your doorway, or even if you have a tree in your backyard you could use a branch. My friend used to do just situps, pushups and chinups starting in grade 7, and I remember in grade 8 when he humiliated a guy twice his size (he was probably 5'2 and the other guy was like 6'4) in a classrom-wide arm wrestling tournament.