Oh dear! Corbyn also went to private schools so why aren't you criticising him too?
According to Wikipedia, McDonnell went to a local grammar school and then "began training to be a Catholic priest, receiving a Church grant to attend St Joseph's College, Ipswich,[11] a Roman Catholic boarding fee-paying independent school...before eventually deciding against the vocation at the age of 15 or 16".
I know that Wikipedia isn't always accurate but it sounds like he wasn't there very long, and maybe it was the best way to train to become a priest at that time.
A vocation to be a priest is surely a noble one.
Why do you feel the need to belittle him for this? Everyone has the right to change their mind.