In her shoes I'd pursue a claim against the school. The 'did it before' thing is irrelevant. Did what before - served food? If pupils choose to eat certain foods (or not) how can that be somebody else's responsibility?
It's absurd !! can you imagine a woman, unwittingly saying to a femail colleague "oh he's hot isn't he?" and the colleague is a lesbian? would it end in a tribunal?
Or a builder pulling up at a site with a MVFC sticker on his van, and the site is mainly CBFC fans? sackings?
The school's decision is so wrong it's anti-cohesive...
Pandering to religious/political groups is wrong in this modern era !!!