If you are approaching a stopped school bus from behind, then stop at least:

• 20 feet away
  Explanation:

  "If you are coming from behind a school bus, you must stop at least 20 feet away when its red lights are flashing and the stop-arm is extended. This ensures the safety of children getting on or off the bus."
• 30 feet away
• 50 feet away
• 5 feet away