I hesitate to respond as this seems likely to be a sham post.
But... (Where to start?)
1) Without doing any research, I'm going to guess that Ohio has its fair share of pot busts. Probably a whole lot more than arrests for playing poker or speeding. You shouldn't underestimate the risk of getting busted.
2) Because your parents may break the law in no way invalidates their argument that you should not. You may consider them hypocrites, but so what? The advice is still good.
3) If you describe you non-smoking state as "sober," perhaps you were smoking way too much.
4) If you're doing worse at school now that you do not smoke, it has nothing to do with the fact that THC is no longer coursing through your brain. Maybe it's a petulant blow-back on your part. Pot will make you think you are more brilliant than you are, but it's a sham. Pot didn't get you A's and B's.
5) I like pot and I smoked it when I was your age. Of course I wasn't stupid enough to let my parents know I did, but I'm not someone who automatically thinks your smoking pot is a bad thing. Nevertheless --- You are 15 and unless you are prepared to live on your own (which I very much doubt) then you need to abide by the rules of the people who make your life possible.
Does it really matter why they don't want you to smoke? You're never going to agree with their reasons. It should be enough that they don't. Like most 15 year olds you have no idea what it takes to raise one of your kind: the sacrifices; the worries; the stress.
You should honor thy mother and father, but even if you don't you should have the personal sense of honor to accept the rules with the rewards. It's pretty clear that you get the rewards, even if you don't realize it. I doubt your parents are making you work either outside or inside your home. In fact it seems the only standard of achievement you consider for yourself is "doing your homework."
This is all well and good, you're a kid. If your parents can afford it, and are nice enough, you shouldn't have all sorts of financial burdens at your age. You will, however, eventually leave home and be out on your own. When that happens, smoke all the pot you want. Don't be rude to your parents, but if you're paying your own way, you get to make all of the choices. You also get to suffer the consequences of all of those choices, but that what being an adult means.
(Still think this is a sham - do kids today ever call it "maryjane?" Did they ever?)
Disclaimer: I am all for the legalization and consumption of marijuana, but I am even more for living by a code of personal honor.