The answer is simple: optics. What organization wants to be associated with child abusers? What organization wants to be associated with any sort of criminal?
Once allegations have been levied against a teacher, the organization wants to create some space between the individual and the organization. Schools will put out statements of horror and disbelief that one of their own could commit such a heinous act. They want to make it clear that the act you are accused of is not condoned by them.
Imagine the headline if it were otherwise, if the administration did defend the teacher: “School District Successfully Defends Teacher Against Child Abuse Allegations!” What message would that send to the community regarding that school? No, remember, the school district is on the side of the school and the students, not the teachers.