They cannot afford to antagonize their customer base any more so than other businesses absent some fundamental issue of ethics or borderline illegality. If this girl was mamash admitted without the proper vetting to the yeshiva’s standards or would have resulted in conflict and brogias with the school’s existing student base and families, they were within their rights to reverse an admissions decision.
No they weren’t in their rights to reverse an admissions decision, unless that girl posed a direct threat to other students. Of course, we don’t know that that wasn’t the case, because details were not provided.