Assuming that you want to get to school on the dot at 9:00, I'd estimate that you should leave around 8:00.
Lets say you do go at 8:00.
You'll walk there an it will be around 8:15. Then lets walk around. When walking around, you will then have the time set at 8:45. Then we'll want to walk to school which will take 15 minutes; you will arrive at school at exactly 9:00.