Because as an officer of the school he was required by law to report it withing a certain time frame. As an officer he can't get off the hook by passing it up the chain.
23 Pa. Con. Stat. Section 6311(c), regarding staff members of institutions:
Whenever a person is required to report under subsection (b) in the capacity as a member of the staff of a medical or other public or private institution, school, facility or agency, that person shall immediately notify the person in charge of the institution, school, facility or agency or the designated agent of the person in charge. Upon notification, the person in charge or the designated agent, if any, shall assume the responsibility and have the legal obligation to report or cause a report to be made in accordance with section 6313. This chapter does not require more than one report from any such institution, school, facility or agency.
Nope. Don't see anything saying that officers of a school are required to report it. "Person in charge" or "designated agent". If there's a case that makes the term "person in charge" applicable to all officers of a school, then please let me know. The Grand Jury Presentment says that the president or head of a school has the duty to report.