If school starts at 9: 00 and you want to go to walmart, when would be an appropriate time to leave? school is about 10-15 minutes away from the house. walmart about the same, but all including the time walking around in walmart would probably be a half an hour.

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. 

Well you also have to look at the time from Walmart to your school. If school starts at 9:00 then you have to leave the house at about 8:00. Once you get everything you need then you leave straight from Walmart then you will get to school on time if the school is about 10 to 15 minutes apart. If you plan on going back home then you would have to leave about 7:45 to 7:50