34 CFR § 300.9 - Consent.
Consent means that -
(b) The parent understands and agrees in writing to the carrying out of the activity for which his or her consent is sought, and the consent describes that activity and lists the records (if any) that will be released and to whom; and
(3) If the parent revokes consent in writing for their child's receipt of special education services after the child is initially provided special education and related services, the public agency is not required to amend the child's education records to remove any references to the child's receipt of special education and related services because of the revocation of consent.
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