Hire an attorney who specializes in special education law. You can find one through the COPAA directory. You will probably need to file for a due process hearing and have your advocate appear as a witness.
You can file a complaint with the state education agency, and you do not need a lawyer for that. You can also file a complaint with the Office of Civil Rights in the U.S. Department of Education (and if school lunches are involved, the Office of Civil Rights in the U.S. Department of Agriculture). You do not need a lawyer for that either. You can request a due process hearing. I recommend that you have both an attorney and experts for that process. It could be you need a different advocate to assist you in dealing with the school. You should go to copaa.org for assistance in finding an advocate (or lawyer). Additionally, at wrightslaw.com you will find a link to the yellow pages for kids that will have contact information for you. You should also review the extensive information at wrightslaw.com to understand fully your rights, and how to question the actions the school is taking.