This was a high school but the American Council of Education has a Rule Book:
An Academic Costume Code and An Academic Ceremony Guide:
Some Permissible Exceptions
Only members of the governing body of a college or university, whatever their degrees, are entitled to wear doctor's gowns (with black velvet), but their hoods may be only those of degrees actually held by the wearers or those especially prescribed for them by the institution.
The chief marshal may wear a specially designed costume approved by the institution.
It is customary in many large institutions for the hood to be dispensed with by those receiving bachelor's degrees.
Persons who hold degrees from foreign universities may wear the entire appropriate academic costume, including cap, gown, and hood.
Members of religious orders may suitably wear their customary habits. The same principle applies to persons wearing military uniforms or clad in special attire required by a civil office.
It is recommended that collegiate institutions that award degrees, diplomas, or certificates below the baccalaureate level use caps and gowns of a light color, e.g., light gray.