During a preflight inspection of a flight school airplane, the flight instructor noticed a missing rivet on the left side of the vertical stabilizer. He also noticed that the skin of the vertical stabilizer was bent outwards. The flight instructor walked around to the right side of the airplane and noticed that the same rivet was missing on that side, and the skin was also bent outwards. He called the maintenance department out to the airplane to look at the damage. They subsequently told him the airplane was not airworthy and to write it up as a discrepancy. The maintenance department immediately grounded all the other airplanes and inspected the vertical stabilizers for any damage. No other damage was noted on the remaining 49 airplanes of the flight school operator’s fleet. It could not be determined when the damage to the vertical stabilizer occurred, but the damage was likely the result of excessive side loading of the vertical stabilizer during flight.