There are fourteen errors in the use of punctuation marks in the following text. Rewrite the text correcting the errors.

Student : May I come in Sir
Principal : Yes come in. What is your problem Student : I want to change my 4th subject Sir.
Principal : Why
Student : Sir Mathematics is now my 4th subject but it seems to me very difficult. So I want to take biology instead of Mathematics.
Principal : Do you thingk biology is an easy subject
Student : Not easy. Sir but easier than mathematics.
Principal : Ok Write an application. Remember your application must be endorsed by your guardian. Otherwise, it will not be accepted.