And then quirrel won a bet with dumbledore when it turned out that first-years could cast the patronus.
Fake-moody's statement that he'd get no more than a bloody nose...You woudln't want to try that in the MoR verse. one of the students might be tempted to test it. and if the've been in quirrel's wargames, well...

you get the idea.

cause in MoR, it IS largely practice-based. in cannon rowling left it vague.