I know his mum a little (not that well), and I'm sorry but I think making him remove the t-shirt is daft. Mandy does feel strongly that mercury has damaged Sam; Sam is severely autistic, and would quite possibly have got distressed by being confused at having to change his t-shirt (in other words am reading between the lines of the report). If they didn't like the t-shirt the school should have asked his parents to not send him to school in it again, not changed it once he was there. I can understand why that might be an issue.
Mandy wears those t-shirts herself as well. I don't suppose she thinks that mercury/heavy metals is involved in every case of autism, but I think in her son's case there's quite a bit of evidence that it has harmed him. She should be allowed to express that really.