Is it legally permissible to disclose salary information even if there is a confidentiality clause in the contract?

Recently, while discussing new teachers for the upcoming semester with some colleagues at school, one of them mentioned that most of the new teachers have 3-6 years of experience. This got me curious about how much they might be paid. Since we are in a rural area, I assumed the salaries would be relatively low. However, our principal mentioned that there is a clause in the contract stating that revealing salaries to other colleagues could lead to termination. Is this legally permissible? While I understand warnings or reprimands, isn't termination a bit extreme? Does anyone have insight on this situation? I would appreciate any opinions.