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

Question

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.