I heard of a school that wrote, very large, in a Sharpie "property of Southern Oaks School" on the outside of the garment. It may have even said "return to Southern Oaks School"--I can't really remember but it was clearly not anything you would want to keep and you'd surely know whose it was!
SHC

How about, stapled ( in an envelope) to the bag with the boy's soiled laundry:

Dear Mom,

Johnny had an " accident" at school today, and you may not realize it, but he's not wearing his own uniform now! __________ ( Nurse's name) would greatly appreciate it if you would launder this " loaner" and return it by___________ ( give them 2 days) so that she has a uniform available when the next student has an accident.
Sincerely,

PTO.. or whomever...

THEN.. if it doesn't come back in 2 days.. send another:

Dear Mom,

_____________ (Nurse's name) has not yet received your loaner uniform. Is it possible Johnny left it on the bus, or in his backpack? Please return the uniform by tomorrow. The PTO spent $500 last year on replacing " loaner" unforms that were never returned. Unfortunatey, because of that, we regret to inform you that if you don't return it by ( 3 more days from now), you will be billed the full $X.00 for the cost of the uniform!

Help. Our nurse gives out uniforms when students have "accidents". Our problem is we rarely get this uniforms back. Last year we spent $500.00 on replacing these uniforms. We have decided to send a cute note home with the uniforms saying that they need to be returned to the school. Does anyone have something like this?