I do agree with both sides of the argument. While she has worked hard for it and is likely to take extra care of it, there is still the chance that someone could take it while she is not allowed to have it. If she is so desperate to take it then she probably will even without permission (I know I would have) so it is better to give her some ground rules that still give her some trust aspect. I would suggest that at the end of the day you ask the teacher if she played with it when she shouldn't have, and quite often they will understand that was part of the deal she made with you on whether or not she could take it, and if she didn't live up to her end of the bargain then she looses that priveledge for a week or so.