Is it permissible for my sister to take a glove she found at school, the owner of which is unknown?

If the found glove has value and is considered property, then its ruling is that of a luqatah (found item). It is not permissible to use it before identifying its owner and announcing it for a year, because Islam protects people's rights. It is also not permissible to take it in exchange for another lost glove, as its owner might not have taken it.