I’m sure the teacher in question has fed dud information that it’s policy to call every parent that.
My better half works at a school in L21 buddy…so I can assure you that legal guardian, primary carer, mum, dad, grandma, etc are all used.
If in doubt, you would not call any person a role that you weren’t sure they were. Ie calling the mum’s boyfriend dad by mistake for example.::