There was a firm in England that made the dummy M1 enbloc clips used to fill out the cartridge belt pockets - but their name escapes me.
They were cast resin.
I'd just load empty brass and leave the primer pocket empty, or drill it completely out.
It's safe - and readily visible.
The Government-Issued dummy for the .30-06 was fluted and had a primer - though some had a primer pocket that had never been opened through.
Later 7.62 NATO dummy rounds are both fluted , with the open-yet-closed primer pocket, 'and' are Black in color - as a ready identifier.
You 'could' find a local National Guardsman who might be able to supply some fired 7.62mm Blank ammunition - then drill out the primer.
Fired Blank brass is collected - but Turn-In isn't as critical as 'real' brass.
Once fired - the 'bullet nose' and the opened primer would then be an open tube - yet would look like a loaded round from the side.
You see a lot of in in close-ups of belted machinegun ammo.
On the other hand - since he has no reason to 'load' a Garand - he can have triangular wooden blocks made up to fill out the cartridge belt pockets.
If he wants to demonstrate the distinctive 'ping' of the ejecting clip - that can be loaded separately.
Then - he can relate the mythical tale of the 'enemy' - (Germans, Japanese or North Koreans) waiting patiently to hear it and charging while the GIs were reloading.
(Damned fine hearing - in the unholy cacophany of infantry combat - and wonderful senses of timing - to be able to pull off an attack as an entire unit was caught 'empty'...)
If he's allowed to bring weapons into a school - then he has an enlightened School District indeed, since many, many gun shows were forced to find other venues because of various edicts forbidding firearms on school grounds that were interpreted literally.
Before he does a 'modern' display - he needs to tell the Principal exactly 'what' will be shown and 'how' it will be demonstrated.
Otherwise, confusion reigns and kids aren't afforded the opportunity to see and learn.
I wish him luck.
Vaya,
Scouts Out!