Much of the tension is coming from the fact that we don't know who the bad guy is. Is it one bad guy doing everything? Is it several bad guys acting independently? Is it several bad guys acting together in one or more groups? Stalker, intruder, threatening messages, hidden cameras, ... there is potential danger everywhere. Classmate who has a secret crush, classmate who's a date rape waiting to happen, bitchy classmates, creepy powerful guy who wants to sleep with the mom, total bad guy who had attacked and raped the mom and who is now back, jealous sister ... even the detective doesn't seem totally legit.
Good stuff. Hope they are able to maintain the standard set so far.