If a student withdraws from a nonprofit, private high school, is the unused portion of the tuition deductible? Can it be treated as a donation? Thanks for your help.

It depends.  If the school was willing to refund the money but you said "No, keep it" then that would be considered a donation and the school would need to send you a letter or give you a donation receipt of some type.

If the tuition was nonrefundable, then no.  This is just a personal expense and is not deductible.