Wow - this is long and I haven't read anywhere near all the posts, so I'm sorry if I'm repeating someone.
But I have no problem at all with public prayer. A prayer at a school (or any) event doesn't bother me. AND I don't have a problem with any particular religion offering the prayer. If someone is not praying to "my" god (as often happens), then I just wait respectfully for them to finish without participating. I like my kids to see such things and understand that the world has many beliefs, and we need to respect them.
If I were to write a letter, it would not be to discourage prayer, but to encourage diversity in prayer. If 70% of the people at these events are of one particular religion, then maybe have 70% of the prayers from them, and 30% of the time ask the others. When encouraging diversity, it would be unfair to exclude the majority too.