Is it legal for a school to deny waiver of days of absence because the diagnosis of the student/patient was not written on the slip?

I need to know where to look at the actual hipaa rule concerning this.

any help would be greatly appreciated. thank you in advance.

A school usually has a duty, required by state public health laws, to report causes of absences in order to track trends and epidemics. Schools are not "covered entities" under HIPAA, and there are exceptions under HIPAA for "covered entities" (such as providers) to disclose information required for public health purposes, etc. HIPAA privacy and security regulations can be found at 45 cfr titles 160, 164 and 165.

I'm not sure what you mean by "deny waiver of days of absence," but this is probably an issue covered by state regulations.