I've always liked this episode.  It's another of those borderline stories that could be cliché if not handled right, and it really isn't in some cases.  You have your standard stock of 80's high school kids, with big hair and lots of jewelry, in a school in a bad neighborhood. The only reason to watch this episode is to see just how good an actress Adrienne Barbeau is.  She electrifies every aspect of the story, and her performance is the reason it works.  Once she's possessed by the demon, she is completely vicious but also compelling in a sly, illuminating way, and a delight to watch.  When she turns on her class and growls to a student, her voice oily and low, "What's on your tiny little mind?", she's radiates a command of the situation that her plain self just doesn't have. The other students leap to attention, so they're not treated the same way. You see the sad realization on the teacher's face, that violence gets more respect than kindness. 

The effects in this episode are another reason to watch it.  The demon of the building, a gargoyle that sits over the entrance to the school, is done well.  It glows when asserting its influence on the kids, and when the student fight causes the teacher to come under its gaze, the effect showing her possession is effective as well.