I did really bad in high school and I think that was because I was educated below my level. We have three levels of education here, the lowest being average.
Due to lack of interest in homework they put me in the middle one. I felt like I was surrounded by morons and everything we learned I got immediately. Nothing was really difficult. When I feel I won't really learn anything from doing homework, I won't do it. I skipped a lot of classes, got quite depressed and turned things in late or not at all. They made me do a year over again and then I realised going on this way wouldn't get me graduated. I studied and.. sort of did my best and graduated with an average 8/10.
Now I have a diary to keep me from forgetting deliverables and homework, and it gives me a nice handle on how much time I have left before I have to turn something in. I can say I'm managing, timewise. Though I still procrastinate a lot.
I put aside the idea that school is mentally challenging (because that was "the problem" back then). I just do it so I can get a nice piece of paper which will bring me somewhere in life. The mental challenge had to be done by myself. Whenever I feel the need to really think, I read a book on quantum mechanics, learn how to play guitar, steampunk my computer mouse or learn a bit of japanese. That's what keeps me happy nowadays. (and also what keeps me from skipping classes)
Also, most institutionalized education only test you on your ability to retain and later regurgitation information, which I find to be extremely tedious and boring, rather than the ability to critically think and formulate an answer based on your own perception of what you've learned, which is where I would usually excel.
I can relate to this. My teachers in high school always said to me that I could do better, because they knew I wasn't dumb. The problem was, I couldn't do better. I suck at reproducing, especially if I'm not motivated at all.