As somebody who knows a collector of WW2 paraphernalia, a German WW2 helmet with its original markings is not cheap. A full authentic uniform would be VERY expensive (especially stuff that fits... most surviving examples were surplus and usually very small - hence it was never issued out and that's why it survives).

Any self respecting collector would never let somebody ride a bike wearing an authentic uniform - the possibility of the grease on the chain hitting the leg of the uniform being an instant deal breaker for me, and I would assume most other people. Its just too ridiculous of an idea that you would lend out that sort of kit for somebody to WEAR all day at school.

That said, I did wear an authentic WWI US Navy uniform for Halloween once when I was a kid, but the difference there is that US Navy stuff is always undervalued and I was still small enough to fit in the undersized surplus stuff. Pants/pullover/hat all in cost like $70.