You gather all the materials necessary to create a time-portal. Now functional, you use it to return to the day of your big test. You disguise yourself and fix the vending machine. Now, Zim won't be angered by it and thus won't knock out the power to half of Devastis. You happily return to the present and check the historical records. To your dismay, nothing has changed! You try again and again to stop the power outage but it's no use.