Chilling Effects for Mid-grade:
If you add the language, it does somewhat definitionally shift from mid-grade to adult or young adult, which means the whole temperature of your story would need to shift, too. There is a weird double standard to writing for the younger audience, where the audience wants edgy, but they also don't. Then the publishers want edgy, but they also don't. This is one of the reasons I don't really want to write in this age target. There is, however, nothing wrong with writing for young adults/adults. Tweens can read/write books for teens, teens can still read/write books for adults, and teens certainly don't like being told to not use bad language (I have two teens, and they recreationally curse to show they're "grown up.")
And yes, my children knew numerous swear words at thirteen, and I was the only one that judged them for it (and your parents don't count). My father used profanity excessively (he grew up a bar-owner's son, then joined the army), so I'm personally a little picky about its use and think most folks just use it for effect rather than to be genuine, so I judge adults for it more than tweens/teens.
If you think your book ends up bridging the space between audiences, then find literary agents that represent books in both categories. They will be able to decide if the story is still appropriate for mid-grade with the language, and they can always say, "Drop the f**king bad language!" and you can edit it out. 'Darn' and 'fracking' are a little ridiculous, but people will know what you really want to say. Honestly, if you get to the point where someone reads your book then the language won't be a deal-breaker if you express a willingness to remove it as needed.