Early one morning, a mother went in to wake up her son. "Wake up, son. It's time to go to school!"
"But why, Mum? I don't want to go."
"Give me two reasons why you don't want to go."
"Well, firstly, the kids hate me, and secondly, the teachers hate me, too!"
"Oh, that's no reason not to go to school. Come on now and get ready."
"Give me two reasons why I should go to school."
"Well, for one, you're 52 years old. And for another, you're the headmaster!"
We often say firstly when we want to give the first in a series of explanations or reasons for something.
The headmaster is called the principal in US English. (S)he is the boss of the school.