Is it legal for the school to allow the police to search my 14 yr. old child and question him without me being present?

One child told the principle that my child was selling weed. The principle then took my child out of class and patted him down and proceeded to call the police. The police then searched him and the dog sniffed everything. They found nothing on my child or in his belongings. One of the office personnel accused my son of trying to shove something in his behind. When the cops came to search him he put the glove on with his middle finger sticking out as if he was going to do a cavity search. My child was very frightened by this. Before the search, the officer told him that I couldn't make it and that he needed to come clean about what he had in his possession. My son told them that he was running late and had an Adderal (ADHD) that he forgot to take at breakfast, he told them that it had fell out of his pants on the way to the office. The drug dogs have searched that path and didn't find anything. My child didn't have anything in his possession, he was scared of what they would do to find something. Now we have to go to a hearing to have him expelled on grounds of having something that he didn't have. What can I do?