Of course, you have to bear it. Whether you are in a rental house or not during the summer vacation, since you two share it, it means that your house is your own. Whether you live here or not, as long as you don't move out, this house is still yours, and your classmates can't take your house, can they? So you should still bear the rent, but you can discuss with your classmate that she will bear the utilities during the summer vacation. After all, you don't live here and you don't use water and electricity.