In the years since learning that her daughter had been bullied on the Internet, Sandy Reeves has made a point of following social media trends and tracking new apps where teens can hurl insults at one another. She knows them all. Or at least she thought she did. The Hanover Park mother was surprised Thursday to learn that several local schools sent letters warning parents about the dangers of Yik Yak, a Twitter-like application that lets kids post anonymous comments to users in a 5-or 10-mile radius.