You may edit the letters however you would like. There are a couple highlighted areas you will need to typ in so it personalizes the letters. They are written so as to not take a stance on a specific proposal. I have my own ideas (and on school funding they align pretty closely to those you will find at http://illinoisvision2020.org). The main thing schools need is a budget for the 2016-17 school year.
Also, do not underestimate the value of a phone call. They do keep records of what people are calling about. You can keep it brief and hit the main idea that a K-12 budget must be approved.