Sorry to be the voice of dissent, but I do not think a teacher should discuss a named pupil outside the school with anyone, let alone someone whose identity you are not sure of. Teachers are privy to all sorts of information some of which is very sensitive. Moreover, in a pub they should be drinking and enjoying themselves. Not discussing work. If you know the teacher well enough, I would approach him via an after-school 5 min appointment and explain the problem he has caused. He will doubtless be mortified and never fall for the same mistake again. If you do not know him well enough to do this, I would take a similar tack with his line manager eg head of year or head teacher, not to be nasty and dob him in it but because he needs to know the consequences, and to learn to do what grown-up eachers do - keep shtumm, use nicknames if in public and above all switch off from work.