I filmed a high school play two nights in a row. The first night I used the built in mic. Yesterday I bought a directional mic. The audio was not really bad. But it was not as good as I had hoped it would be. The directional mic seemed to really clean up the audio but it sounded kind of muffled.
They had a sound guy there but he had no idea to hook me into the system. Can this be done in the future? What would I need to do this? The school had mic's on most of the kids and other throughout the stage. I don't own a wireless and don't think it would have done me much good anyway. Anyone have any suggestions? Thanks.

Connecting to a sound mixing board is a common solution. How to make the connections depends on the details of the sound board. Some offer a tape recorder output at consumer line level. Some offer XLR outputs at either line or mic level. You will need that detail to proceed.

Some folks use an external recorder (e.g., MiniDisc recorder) connectd to the sound board to record for synch in post. That can work well and avoid haveing to run wires from the sound board to your camcorder.

Wireless can be used by placing a directional mics at the edge of the stage to pick up the actors (it reduce audience sould lvels). (I use them for recording marching bands on the field at half time.) Shotgun mics, especially low cost models, can change the sound character significantly.