Officially every minor or other person without legal capacity must have a guardian, and in said relation remains a ward of that guardian. Now, the guardian may a biological parent, adoptive parent, a caretaker of institution where the person remains, or any other such entity. This is the legal position. It's extremely dry but if the school wants to remain completely proper e.g. in relation to orphans who have only legal caretakers, but no parents, they might try to use the phrase guardian, and respectively ward. Of course if they use parent's ward, they are simply making a mess of it, trying and failing to sound professional.