Try mini magnets. You can get some pretty small strong magnets from Hobby Lobby or Michaels or other such craft store. Make sure you get the little silver ones as they pack the most punch in the smallest package the larger blackish ones are not very powerful and are pretty large. Either sew them between the layers of the placket or glue them directly on the shirt. When you get them places right they are very handy, quick release and sturdy.
The old fashion method would be to baste the shirt shut and pull it off vigorously at the right moment, however, it sounds to me like you need a "Stripper Zipper."
Remove the stop from one side of a light-weight jacket zipper, and set it into the shirt under the front tab facing. Use a hook and eye to keep it shut. At the "moment of reveal" unhook the hook, the zipper comes
apart and the shirt opens.