The issue with external walls of schools has been discussed in this clarification:
The non-existence of acceptable POIs are not a reason to submit/accept something in lieu.
If what you say is true, this is a point of improvement for your local community. Each object is reviewed according to the details and context. While a mural may be meet the eligibility criteria, the location being not in the allowable locations should garner a rejection. This has been told in the first sentence of the rejection criteria page. You can request for removal for murals with the similar context.
The reviewing process is not perfect which is why there are incorrect approvals and rejections in the first place.