This isn't about erasing women, or anything to do with trans issues. Good god! It's about children who, due to death/ divorce/ abandonment/ being taken into care/ mum in prison/ etc./ etc. are not living with their mothers.
After all, it's well-known that Mother's Day can be difficult for adults|, who have lost their mothers, have a bad relationship with their mothers, or for women who are themselves childless not by choice.
I've come across it more often, in fact, with regard to Father's Day, as there are many more children growing up without a father in their lives than without a mother in their lives.
Perhaps the school could call it Mothers' and Special Persons' Day- so that children can celebrate their mothers, but those without mothers are not excluded, and single fathers, stepmothers, grandparents and other kinship carers, and foster parents can also get some deserved appreciation.