“But they were responsible for enforcing a school policy meant to protect the safety and well-being of the other students.”
Actually, it is more likely this vilification/demonizing of anything fire-arm related is part of an effort to erode support for the Second Amendment. Raise generation(s) fearful and/or hating firearms and ammunition then the gun-grabbers win.
What you say is true but the issue with the spent shell casing itself is not a matter of the 2nd Amendment. I can see 1st and 4th Amendment issues here but where the shell casing itself is harmless then the object is equal to any other harmless object. Then the question is best considered by determining what purpose is accomplished in punishing a child for possessing a demonstrably harmless object.
That purpose being to advance a political narrative on the part of the school.
Therefore the child’s freedom of speech and right to be secure in his possessions have been violated. But his 2nd Amendment rights have not been violated as it is generally accepted by the courts that children do not hold such rights.