My 12 year old ASD dd got herself into a bit of a state last night. They are starting hockey in PE and a mouthguard is required.
I bought one from a sports shop that you heat and mould. Even though I trimmed the sides so it doesn't go so far back, dd found it unbearable, she has a lot of sensory issues.
My oldest dd (not on the spectrum) also wasn't overly keen on using hers but managed to get away with keeping it in her pocket and put it in when the teacher was looking 
I'm not sure whether to tell dd to do this or speak to the SENCO? Are they allowed to not wear one with parents' permission?