Work Shown:
time = distance/speed
time = (1.5 miles)/(3 mph)
time = (1.5/3) hours
time = 0.5 hours
time = 30 minutes
It takes 30 minutes to walk to school, so you should leave a half hour before 9:00. Subtracting 30 minutes from 9:00 gets you to 8:30. If anything, you should start a little bit before 8:30 just in case something might slow you down for one reason or another.