Your result for acceleration due to gravity,
Since initial velocity is 0m/s, the equation becomes
You should get a value closer to -
I would recommend that you practice timing the drops until the times for the drops are consistent. (Important: Make sure the individuals doing the dropping and timing are at eye level with the top of the table or whatever you are dropping the marble from.) Then drop the marble three times for three trials to get an average for your time. If any of your times is way off from the others, throw out that one and repeat the drop.
I think the major issues are making sure the drop times are consistent by practicing first, before actually recording any data. If the time remains around .43s, and you are sure of your height measurement, then I would propose that air resistance is affecting the acceleration of the drop.