My ex moved house last year with out telling me the new address and moved child to another school without my knowledge. I discovered the move by chance when Bank advised that a joint account had been closed and sent me copy of letter they had sent to ex's new address.
However, if ex moves again I may not be so lucky? I don't think ex would change the child's school again as it a highly rated, but is the school obliged to give out addresses in the event of another move? They would likely want proof that I am parent. I have copy of child's passport, but not birth certificate.