It IS an empty brass tube! Why does it matter how it was emptied? You surely don’t believe the BS that you spewed here?
All the teacher saw was a 7 yr old with a bullet. That teacher probably wasn’t a firearms expert, and certainly didn’t know the origin, status, or provenance of the “empty brass tube”.
But they were responsible for enforcing a school policy meant to protect the safety and well-being of the other students.