It doesn't seem to happen so much now, but at one time people would give their child two christian names, call them by the second one, and expect you to know this. Why??
Children are registered in school by the names written on their birth certificate, so if the name is Joseph that child would be entered as Joseph in the school register, a legal document, and the teacher would address the child by that name before she got to know him. She would also make labels for drawers, name cards, coat pegs etc. in the holidays before term started, so if a child is unable to recognise an alternative form of his name it would help if the aggrieved parent notified the school office of the preferred way to address him, or perhaps went through the very difficult process of changing his given name.
So much easier to blame the teachers, though.