What does your 'gut' say about charging them?
Of note is that you volunteered to take photos for them. The word 'volunteer' connotes 'freely given/provided/performed' (except in the military). Although the employees of the art school receive a paycheck, volunteers don't. I regularly volunteer my services at several charitable organizations in the area and never charged them (or anyone else, for that matter), a penny.
That isn't to say I'd never charge a non-profit. There are some non-profits that have exceedingly large cash flows, such as political candidate organizations. Yes, many who support that candidate are happy to provide their time, services and skills for free. Some even stand on street corners holding placards for that candidate starting several weeks before the elections. But the suppliers of the office space, furniture, utilities, computers, food, TV time, etc, all expect to get paid.
So, was your volunteering intended for financial gain? How about settling for photo credit? One never knows when a potential buyer might see your work and hire you.