Wow, what an icky song to have to analyse. As much as I love Pink, not great belonging analysis material.
I wouldn't really consider it a pun or idiom...cliche is reaching a little, but perhaps...if anything, I'd say an incorporation of slang/colloquial language is used to encompass those who feel disenchanted from formal institutions such as a school. She then encompasses these people by the use of the second pronoun "we" which shows that non conformists can still experience a feeling of inclusion. She accumulates this idea later in the song when she refers to them as "underdogs".
Anyway, another pro tip for songs, is you can't just analyse the lyrics, you are meant to look at the actual musical quality of the song too i.e crescendos, tempo etc.