Are teaching assistants allowed to take a class??

Q1 My son's class teacher was off sick today and the TA took the class (reception), there was also another TA, a one on one teacher (for a particular Child) and me (parent helper - I didn't see this as a problem but when I mentioned it to a mum she said they aren't allowed to do that. Anyone know if that is the case? I'm just curious Lets hear your views on this. Thanks x Solidarity