I have the utmost respect for anyone who can teach adolescents without murdering them, so a certain amount of grumpiness at this time of year is understandable.

As for your manager - others have said it - she's a moron. One of the very first things I learnt about having staff is that happy staff are productive staff. Unhappy, pissed off people will not go the extra mile for you when you need them to work late to get something important done.