I know your problem. You’ve made two mistakes. First, you used
your classmate’s pen without her permission. Second, when you broke the pen, you
didn’t tell her the truth. Yes, you must tell her immediately. If you tell her
the truth, perhaps she’ll be angry with you, but at least you’ll prove you are
honest.
Then I think you should offer to pay. You can use your pocket money
to buy a new pen for her. She will realize that you’re truly sorry.
I hope
the advice can help you. I’m sure you will make up with her soon.
Yours,
Tom