Did I violate my clients right to due process?

I operate a LLC that is state-approved to validate Montessori schools. I conducted a training at a school that my company validated in mid-July and found several areas where they were out of compliance. It is stated in my validation application and documents that a school must comply with ALL standards before being awarded a Certificate of Validation. After my visit I called the school anonymously and asked questions pertaining to the program. The answers I received confirmed what I''d observed... which is they are not compliant with the validation standards. As a result, I notified the school that I am nullifying their validation based on these inconsistencies. I also notified them of our appeals and complaints process in the event they wish to do either. The owner of the school says I did not grant them due process. He was well aware of the regulations and standards throughout the validation process. Am I missing a step here?