In reading this question over, I think the interests of the school are only served if the best interests of the student with special needs are met. No future financial support by a philanthropic family will outweigh failing to meet the needs of this child. During the admissions process, as the school's awareness of the child's unique needs became evident, it would be critical to have a complete diagnostic work-up of the student and his/her learning profile. This already builds partnership with the family and demonstrates the school's interest in working with the family. If the school does not have the necessary learning service professional available, I wonder whether the family could be enticed to endow a position to address the special learning needs.
I know that even in our school, where we are fortunate to have a learning service department with four professionals, we have had to ask a small number of families to provide an aide to accompany their child and to support his/her learning. In my opinion, the bottom line is that the promise of generosity will probably not be realized if the child is admitted and as time goes on, his/her needs are not met.