I have no idea how or why the writers thought it was a good idea to bring back Krissy. The character-especially the actress-do not work well with Dean. The prior Krissy episode-Adventures in Babysitting-was at best lackluster and the back end of it was some of the worst stuff in season 7. So this episode already has a lot working against it. There's just no life or chemistry to the Dean, Krissy interactions. The entire thing falls on its face because of that.
Even worse the script is far too flippant and nonchalant about the premise of the story. The normal kids who are being trained as hunters does not really work and there's too little material to really sell the concept. The big reveal was guessable within 10 minutes of the episode and frankly everything just feels so utterly forced. This has a childish tone-as in it seems more overtly aimed at children-that is at odds with the show.
Episodes that have strong moralistic themes-After School Special comes to mind-require a lightness comedic tone to balance the cheese and this episode forgets that.
It is only a tender mercy that Krissy is never heard from again. I hope a vamp got her.