Take this with a grain of salt but I would say swimming is probably not a real good option for building endurance for wrestling -- two reasons. One is, you're inevitably gonna have to practice your strokes to be able to swim decently, and that's one more skill to worry about learning. The other is, when you swim, you aren't free to breathe any old time you want, because your head is underwater much of the time -- so you will probably try to pace yourself and not go all-out, especially at first.
I would think a much better option would be intense interval cardio, like doing burpees in sets of 20, running up several flights of stairs over and over, sprinting, etc.
Also, if you haven't been trained in throwing boxing punches, you can do a lot of damage to your wrists if you just start wailing on a heavy bag, especially if you aren't using hand wraps. That said, it might be worth looking into local boxing gyms (real boxing gyms, not "cardio kickboxing" classes) and see if you can take lessons for a month or so, to get familiar with the proper technique. Heavy-bag work is a great cardio option on days your legs are so whipped you don't want to run.
Finally, if you're absolutely sure your school doesn't have free weights, why don't you ask your coach if he can get some? Your school's gotta have money for athletic equipment, right? If not, maybe your team can hold fund-raisers (car washes, garage sales, whatever) to raise a few hundred bucks. Used barbell sets are widely available, and all you really need for starters is an Oly bar (or more than one), a few hundred pounds of plates, a squat rack and a bench with uprights. Check Craigslist, garage sales, classified ads, used sporting-goods stores, etc.
