Most of the rules we live by are not codified. In fact, if a particular practice is officially made illegal through codification and requires enforcement, the rule is weak because the presecribed practice does not come naturally to the collective.
A rule requiring students to wear IDs on campus is weak because it goes against the natural tendency of students not to want to wear their IDs for whatever reason. Even with guards posted at strategic points on campus, students sometimes still walk around without IDs or with IDs borrowed from their friends. The long list of students violating the ID rule is an indication that the ID wearing rule is weak. (Moralists beware: I am not saying that the rule is wrong. I am just saying that the prescribed behavior is not natural)
A lot of rules that are not codified are strong. There was a time when I sat on a committee tasked to institute a dress code in the University and one member suggested that there should be a ban on garish jewelry. My reaction back then was that students never wear garish jewelry implying that it must already be an unwritten rule among students that they should not wear garish jewelry.
This afternoon, while waiting for something at the administration building, I saw two girls wearing skirts from some school with a uniform. I realized then that I almost never see female students wear skirts in the university where I work. It must be an unwritten rule that discourages them from wearing skirts. I remember girls being teased, "Wow, mukha kang babae ngayon" if they come to school in skirts.
Some rules need not be written and these have become very much part of our taken-for-granteds. These unwritten rules are more powerful than any security guard lurking behind a post waiting for unsuspecting students who fail to wear their IDs.