Boston just had its first 70-degree day, and with the warmth of spring comes the magic of prom. In this case, however, prom isn't so much magical as murderous. Gasp! Opening this evening is Prom Night, a probably-not-so-Hitchcockian thriller we expect will be centered on lots of shots of a scantily clad, excessively frightened Brittany Murphy Spears Snow (it had to be one of those). The real horror of this genre is its tendency to......