Buffy The Vampire Slayer S02E12

Bad Eggs

 For Health class, students unknowingly adopt "eggs" of a Bezoar demon growing under the school. Its arachnoid "babies" are able to directly control a person's central nervous system. Dealing with a pair of outlaw vampire brothers in addition to slaying the enormous Bezoar before its progeny take over Sunnydale, Buffy gets ultra-grounded for her trouble.