Near the end of season 2 in Teen Wolf (TV Series), Stiles shared the story of his friends with Ms. Morrell, even the secrets he never told to anyone else. So why did he share it with a person like Morrell, who is unknown and not trustful yet. Even though he doesn't previously know about Ms. Morrell's awareness to the supernatural things. So why did he reveal this to her?
After re-watching season 2 and season 3-part A, it looks like Stiles just needed a person to share his feeling towards the supernatural events with. He was not comfortable in telling this to his father and doesn't have anyone else to share his stuff with. And Scott was also stuck in his own problems, so a school counselor looks appropriate to him. He doesn't know much about the school counselor but just shared his thoughts.