Contractual terms are generally interpeted in line with their plain or commonsense meanings, when the agreement itself does not provide a more specific definition for this situation. A "website" is not third-party social media platforms: it is a site owned or controlled by the school, for school purposes. It is likely that if there was a disagreement, complaint, or conflict, that it would be found that consent only to the school's website would not include third-party social media platforms. It would be a good idea for your school to revise or supplment its consent form(s) to include social media platforms.