[MayflowerMadam]

“No typical child who is 12-13 would select that tee-shirt on his own.”

OMG... you don’t know my home-schooled nephews, all under 12. They’d choose that in a heartbeat.

[Blue Jays]

"...you don’t know my nephews..."

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This is a terrific learning opportunity for this young kid about decorum, etiquette, and restraint.
Reward the child for being a freethinker. Maybe even offer to buy him a second tee-shirt to wear with pals.
Then instruct him he WILL wear a plain oxford shirt or plain sweater on the school field trip and not make a spectacle of himself.