How Can Abuse of Special-Needs Students in School Be Prevented?

The media has been full of stories of children with special needs being abused in schools, by teachers, aides, or other students. Each story is more disturbing than the last, with distraught parents threatening lawsuits and criminal charges. How is it that a place parents trust, where they send their child in good faith, hoping to get them the help they need, can turn into a place of abuse and cruelty?