(AP) – U.S. schools have bolstered security in the aftermath of the horrific mass shooting at an elementary school in Uvalde, Texas. Some of the changes include making visitors ring a bell and temporarily banning large backpacks. At least one district is ending the school year early. Administrators are especially jittery as more details about the shooting on Tuesday come out. Authorities say it took officers more than 45 minutes to confront the 18-year-old gunman who killed 21 at Robb Elementary School. The larger police presence at schools and a rash of copycat threats have only added to anxiety levels for students and educators.