Measuring acceleration due to gravity

I came up with a complicated way of measuring acceleration due to gravity so that the class would have to use acceleration = change in speed/time taken rather than have the computer do the calculation for them. I used 3 light gates. The computer timed the fall from the top light gate to the bottom light gate and that came out at 0.281 seconds. Another light gate at the bottom was attached to a datalogger that measured the transit time of a 10cm card at the bottom, which was 0.028 seconds. The speed of the card at the bottom was thus 0.10/0.028=3.6 m/s. It reached that speed from standstill in 0.281 seconds. Acceleration = change in speed/time taken = 3.6/0.281= 12.8 m/s/s. True value is 9.81 m/s/s so we were out by 30%. That's a bit high!