She is just moody like that.
She is a naughty girl. (or likes acting the naughty girl.)
She likes acting the bad girl (disruptive, here. Not meaning bad to the bone!)
It´s her notion/idea of fun.
She does that on purpose, just trying to bother (others/the teacher etc).
She is just plain irritant (wether on purpose or not.)
(It) looks like her teen hormones have gone haywire!
Of course, these need context. It requires you think of the intended meaning in a given situation.