Thank you for the additional information.
You do, essentially, need to make a specific issue order application.
This is the application to make where there is a disagreement over school choice.
The issue is separate to the time you spend with him though, of course, and his mother stopping contact because of this disagreement is something the court would frown upon.
The school in question should be able to keep the place on hold on the basis of the information that there is a court application in place to decide the outcome of the disagreement.
You can be firm in reminding the mother that disagreements between adults should not impact on the time you spend with your son and invite her to reconsider this decision before taking the matter to court.
I hope that this is helpful. A c100 application is the one you need to make for a specific issue order.
Can I clarify anything more for you?