Ask those who pulled the prank of adding sugar to your tank how much was used. About one teaspoon of sugar dissolves in 15 gallons of gas. Move to Step 2 if a full bag was added or if you do not know how much was used. Move to Step 3 if you know that the amount was significantly less.
Take the car to a mechanic and ask for the gas tank to be removed and cleaned. Remember that sugar is more of a nuisance than a chemical problem, but particulates can gum up the works if they make it past the fuel filter.
Do not take the car in if you know the prankster only put in a minuscule amount of sugar, because the heavier crystals settle down to the bottom of the tank, with just a tiny percentage dissolving into the gas. Have the fuel filter changed regularly to get rid of sugar particles that may have made it down the fuel line.