Apparently she's not very up and up on the ganj world, and never was as a kid, so she really didn't notice, I'm guessing there wasn't too much in the brownies. Enough to fail a test though
Sadly, in the real world its never that easy. In an ideal world yes, but how many times do you think they have heard excuses like this. I have seen people lose there jobs over things like this, it sucks, but theres most likely no way to get her job back....