Did you call the law guardian? What did say?
Sounds like mom pulled a fast one as either of you can enroll child. Go to her old school and get copies of when child was withdrawn, then you can see when mom did this.
If school starts in 2 days, going to have to move fast one Monday to get emergency hearing for school district. You can file yourself monday morning for ex parte hearing to have child return to old school. Then if judge grants it , you can use that paperwork to enroll child back into your school.