What is less widely known is that John McDonnell also attended a (Catholic) private school in Norfolk (St Joseph's College, run by monks - De La Salle Brothers), on an LEA grant, as there was no Catholic grammar school in the area. He has tried to pretend he was, (like Stalin, actually!) studying for the priesthood, but this is rubbish. No seminary takes people under 18. It was just a private boarding school.
In a way it's fair enough. He was obviously recognised as grammar school material by the LEA and in those days they had no compunction in outsourcing to a provider of the right religious background for his family, if they could not do it themselves. But he has hidden it, because of the obvious clash with the anti-private school ethos of the party.