obviously by the time it got to the point she'd been fired, the issue had been raised to where she was saying "some kids aren't getting ANY FOOD."

So would the action then be to continue doing it? mmmm no. not by your reasoning, anyways. Unless the behavior was just so ingrained in the teachers that they thought "oh I'll just take a little LESS..."

And regardless, unless why is this worth firing someone over? Yes, if they persistently plagued the principal about it despite being shown by investigation that it wasn't true...but apparently this only came to the principal's desk twice, and only multiple times because the principal seems to have brushed it off without looking into it in the slightest (at least, she's not claiming to have looked into it).

Given that this woman was not actually a "teacher", but a mentor hired from the an outside organization, I could easily see this as being a "teachers stand together" situation. The principal knew it was happening but when the whistle was blown she pretended it wasn't. When the issue was pressed, instead have any sort of truth about "her teachers" brought to light, she fired the whistle blower. Not saying that's what happened, but it could have. Particularly since the issue was seemingly not investigated before this point.