My high school runs a yamaha m7cl and we have around 26 inputs (16 lavs, 6 overheads, 4 boundaries) and the automation certainly makes life a lot easier. The problem with renting a board like that, which i believe was stated before, is that learning how to use it in a short amount of time is hard. In my opinion anything you can do to reduce the amount of mic changes and switching on and off by the actors will be useful since actors (at least at the high school level) are stupid and when they have to do anything to the mics they tend to screw them up. While the yamaha's are great if your working on a budget they can be quite expensive, I'd reccomend checking out the tascam dm4800 which, at $7,000, is significantly cheaper than the m7cl ($25,000) even though the tascam isn't nessecarily a "live board" it should have most of the features your looking for including midi functionality.