My DS did this also (pre-acceleration). He would also sneak his book under his shirt and take it with him to the bathroom. Lord alone knows how long he would 'hide' in there and read (if what he does at home is anything to go by it could be over half an hour!)
(He also does this at home by the way, whenever the thankless chores need to be done.)
I discovered this when I popped in (on the spur of the moment) to help out with reading groups and he wasn't there.
The teacher was just so busy she didn't notice the length of time he was away!! It didn't help that she was letting him read in class whenver he felt like it as (in her words) "he already knows what I'm teaching".
So maybe boredom is the cause?
