1) I suppose so, but it doesn't sound very flattering: 'old student'.
2) It depends what they want to say. There isn't a set answer. Possibly 'thanks, it was a big shock' or 'well, he was quite old' or 'huh! You wouldn't believe the vet's bill!'
3) Grin and bear it - an idiom meaning 'to put up with something unpleasant with good humour'. I decided to grin and bear it. You can't