It’s all good. My high school had crap for a gym too. It was open for crap hours too. Not that I lifted when I was a frosh…I wish I would have started then. You’ve got good resources here, I hope you make use of them.
Barring getting transport to a gym, buy the weight set they sell at most big sports stores: Oly bar and 300lb of weight. You can do everything but squat and bench with that. You can squat if you can clean it to the front position, and you can floor press with the barbell, which will work well in combo with the military press. You can do everything else with it but chins, and I bet you can find a ledge to grip somewhere (or tree).
Find a way to learn the olympic lifts, from descriptions on this site, or some way. Nail the technique flawless for the hang snatch, hang clean, and front squat, and you’ll be MILES ahead of most of the college age jackasses around my gym. Then when you’re a senior in HS or getting into college you’ll be a beast.
Also, you can curl the plates by gripping them through the center hole in place of db curls, and curling the barbell with one hand is useful too…but I agree with the others–you should spend almost all your time on compounds. With enough food, they’ll make you grow like a weed, and your arms with the rest of you. Besides, you can eventually humble all the other high schoolers by hang snatching what they bench press. That could be fun.
There are no three easy steps to building muscle. It takes discipline, perserverence, balls, and LOTS of thinking and planning. Given those four qualities, you can do anything.