I don't know if writing the letter is a good idea or not, given your desire for a long future at the school, but if you do, I strongly suggest letting other people read it first and heeding their advice about wording, if they have any. When we are close to a situation we often find it hard to express ourselves neutrally.
If the school is very specifically play-based, though, I would bet it is impossible to get them to do reading and math. Many parents are looking for that play-based environment. I had more trouble with the NON-play-based aspects of my DD's preschool experience, because they were too easy. The rest was great.
