Hate to be negative about it, but middle school isn't some walk in the park. Even if the material isn't getting harder or anything it's just a really, in my opinion, rough time to be alive. You learn a lot more about yourself, have difficulty finding people that you feel like you can be yourself around (especially since everybody is trying so hard to be like everyone else) and. it just sucks. Not to mention the whole hormone thing.
But anyway, for your own sake, and my vicarious living, do your best to keep your grades up, and be involved with the right people. Do NOT WORRY about your grades though, don't create unnecessary stress. The same exact thing happened to me in middle school -- i dropped a letter average. Everything was made up for by the end of High School though. My point is you still have plenty of time to make up for 2 or three years of *minor* slippage.
So please, do your studies, and keep your head on straight.
Good luck <3