How was Auggie able to get out of school and go home?

Auggie goes to the school nurse, who promptly calls his mother. His mother picks him up without question.

Then I went to the nurse's office and told her I had a stomach ache, which was true, because I felt like I'd been kicked in the gut. Nurse Molly called Mom and had me lie down on the sofa next to her desk. Fifteen minutes later, Mom was at the door.