Unwritten Law
Unwritten Rule, Common Law
noun
This not writing law is a law that hasn't been written down. It's an unwritten law, like taking your shoes off and showering every day.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
学校では日本語でしか話してはいけないという不文律のようなものがあります。
There is some sort of unwritten rule that says people are only allowed to speak Japanese in this school.