Why do you have to tuck in your shirt before entering the school library?

Answer 1

This is something specific to your school and/or your district. It could be anything from an old tradition, or merely to show respect.

Or to acknowledge that you're entering a place a learning, etc. The theoretical reasons go on, I'm sure you could do some non-internet investigating yourself.

Answer 2

Tucking in your shirt before entering the school library is often a part of the dress code to maintain a professional and respectful environment. It helps create a sense of decorum and shows consideration for the learning environment.