So, you've posted about some things here that pose some real dangers in a couple of ways.
1) Choking or any kind of "breath play" or erotic asphyxiation is very dangerous and isn't something that's advised by sexual safety folks for people of ANY age to do (separate from simply holding your own breath like you might while swimming). It's not safe for anyone to be choking you, in any context, ever.
2) Waiting to go to the bathroom when you have to is also not a healthy thing for anyone to be doing, especially anyone with a vagina: that's a way to get urinary tract infection (which then means having to go get healthcare for it, btw, since they don't just go away).
3) Engaging in sex play at school is also a very unsafe thing to be doing in a lot of ways, particularly given it sounds like your school is highly unlikely to be understanding about your orientation OR the kinds of sexual play you're engaging in. In the event you get caught, or word gets to them from other students, and your parents are informed, that also means being outed to your parents about all of this, and certainly not in the ideal way, to say the least. To boot, that also potentially nonconsensually involves someone else in your scene should they walk in, which is just never cool to do, again, no matter our age.
So, that's all that stuff, which I'd advise you reconsider right away. Some of that stuff suggests that one thing you're having a hard time with, and your girlfriend, too, is good judgement around all of this, which isn't super surprising. It's hard to have amazing judgement at 13, which is why you may be already getting ahead of yourselves here, you know?
It also sounds like maybe neither of you have been researching much about physical or emotional safety and BDSM. I'd strongly suggest that before you move forward, you take some real time to do that.
It sounds like you haven't actually looked at the site as a whole so much as the message boards, do I have that right? You talked about other people's problems, which doesn't sound like you were looking at site content so much.
Here are a few articles I think would be a good idea for you to take a look at to start with, and from the sounds of things, your girlfriend, too. Perhaps you could look at them together:
• Yes, No, Maybe So: A Sexual Inventory Stocklist
• Well, F*ck Me! It's a Scarleteen Zine!
• Ready or Not? The Scarleteen Sex Readiness Checklist
• This piece is written explicitly for people with disabilities, but also has some great general information for everyone: https://www.scarleteen.com/wheelchair_b ... disability
• 50 Shades of BS - How to Tell the Difference Between Kink and Abuse
That's a bunch to start with, so how about you spend a couple days with those and then we circle back from there once you have some more basic info on your plate to inform these conversations?