The student was making a practice solo flight to prepare for her private pilot certification test the following week. After an engine runup which resulted in an excessive mag drop on the first attempt, she attempted a takeoff on runway 26. She had used up about half of the 3,500 ft runway before the airspeed indicator furnished a reading. The reading did not get above 40 mph during the takeoff run. The engine started running rough and she decided to abort the takeoff. The airplane veered left off the runway, 183 feet from the end. It continued over the grass surface and into the river/lake surrounding the airport. The skid marks from the application of the main wheel brakes began 215 feet from the end of the runway. The throttle was found in the closed position. The exhaust valve for the number four cylinder was found stuck in the open position. Inspection revealed the airplane was 10 hours past due of its 100 hour inspection.