Once someone becomes 18, they're legally an adult and can do whatever they want to - joint he army, get married, vote, etc.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
I agree with the previous attorney's response. However, typically the student can consent to disclosure of records/information to his/her parent(s). The consent should be specific, in-writing, and directed to appropriate school personnel.
Disclaimer: Please note that this response does not create an attorney-client relationship. The words set forth above are not legal advice and should not be construed as such. The person(s) posting the question should not rely on this response as legal advice. Each case is unique, fact-specific, and should be reviewed in light of all state and federal laws. The person(s) posting the question should seek an in-depth evaluation of the case by a an attorney licensed to practice law in his/her state who can review all relevant facts and documents.