In addition to the crowds of middle-school-aged fangirls who exist to fawn over the primary love interest, there are occasionally individual fangirls as well who shout things here and there.

In this scene, two fangirls, presumably friends, run past the shot, down the hall in a hurry to beat the other fangirls to kendo practice (to watch the guy they are interested in play) so that they can get a good view.

Most of the fangirls in the school are assumed to be seventh graders, though there are plenty of eighth and ninth graders interested in this boy as well. 

Any female voice that can pass as a young teen or teenager and has sufficient enthusiasm should work for this role!