Do not be disappointed if there are not a lot of shops available in your location. Be careful. Choose shops that make money. You are not doing mystery shopping to spend your own money You should be making an acceptable amount of money after you parked your car and finished the reports.
Figure how much gas and time it takes.
Group several shops on a route so each shop makes the total mileage for the route less than if you just took one shop (1 shop divided by one (cuts down on your profit) or x amount of shops divided by x amount of the round trip to do them all equals x amount of miles per shop.)
Total the money you make on all shop and the hours you spend performing the shops, deduct the expenses and divide the net money by the hours. Would you work for that amount per hour?
Would your school work suffer if you spend time on reports rather than school?
Extracurricular activities look great on your resume, shows your interests, gives you contacts in later years. Do not give them up for mystery shopping.