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"need to know" and my child's right to medical privacy... they were distributing such information to other parents... I basically assume that unless I specify confidentiality, ANYTHING that I put into an e-mail is at least hypothetically going to be circulated...
Disclaimer: I am not attorney and this is not legal advice:
Thinking of sharing information under the "need to know" principle brings FERPA to mind. The Family Educational Rights and Privacy Act (FERPA) is discussed online at ed.gov (U.S. Department of Education) and uses the terms "school records", "legitimate educational interest" and "school official" which are to be defined by the educational institution. There is language about information obtained from others not being protected (this may apply to e-mails from parents?). FERPA also discusses access to student records, requesting to amend student records, not requiring student records, and allowing the destruction of student records.

Being open records, anyone may request public school e-mails. Some institutions may redact the name of a minor, some may redact all personally identifying information. FERPA may be one of several laws/codes/policies to consider.

That being said, information posted on a public forum may also be widely read, e-mailed, and otherwise circulated.