Interesting chapter that picks up right after the first one. I agree that you've got the relative ages of the Weasley boys off, the Twins are younger than Percy (at least I presume that's who they were picking on), not older. Personally, I'd spell it out as Shall Not instead of Shalt, simply to make things perfectly clear to dunces like Percy.
One other word bothered me, I do believe "...such a reamand..." should be "...such a reprimand...". I'd certainly be interested in being proven wrong, though.