Homeschooling, as mentioned above. Also allows you to play with how well the kid is caught up.
Though, IIRC, homeschoolers are subject to standardized testing, so "dumb as a box of rocks" won't fly any more than it would in mainstream schools. On the other hand, maybe the kid's parents were rocket scientists and/or hired tutors to bring him up to Einstein levels.
Also, depending on the age, maybe his family applied for hardship so s/he could drop out and go to work, and now that the situation has changed, s/he is going back to school. (That may not apply for younger kids, though; that's actually why we have truancy laws, according to my dad.)
If s/he has a chronic illness, it could have flared up and then gone into remission; it's not the same as getting over it, but for plot purposes, it might work.
I think homeschooling is your best bet. There are many reasons parents either choose to or have to homeschool, and equally as many reasons for the kid to be put back in. In my case, Mom had taken me out of public school because I wasn't emotionally suited for it, but then had to put me back in after a year because Dad got hit by lightning and so she had to work. (Yeah, it went about as well as you might expect. Oh well.)
