student struck by teacher

My son is 13 and is going to summer school. It was the second day and almost reached 105 degrees while he was in school. the school does not have AC. He was asked by a student next to him if he could use a pen/pencil. he said yes and handed him the pen. At that point the special education teacher, (which is an extra in the class due to the absence of the main teacher) came up behind him and struck him in the back of the head and proceeded to tell him to "stop talking." he did nothing in response but do as she asked. When he was picked up from school he was very visibly upset and finally told what had happened. I brought him into school the next day and went straight to the Principals office. We sat and I explained what my son had said, and asked if he could investigate further. Meanwhile i did advise him that my husband has filed a report with the city police and they would be getting in touch. he called later and asked if we would be interested in talking with the teacher, she wanted to have a conference. I agreed and said we would be at the school at 11 am the next day. The morning of the meeting I got a call from the principal stating that a meeting is no longer necessary he had talked to the superintendent and they both agree that talking to the teacher and enforcing the no contact rule was enough and there was no need to go any further. he then proceeded to tell me that he interviewed three other students and one said yes he saw her hit my son, the other said he did not see anything, then told me he asked a third and he said yes but he was not in the room. I asked why he would ask him and how do we find out at what time was he sent out of the class? He had no response but to say that it sounded like my son was being bullied into saying she hit him. I explained to him that it did not make any sense and would like to see what they had to say together in the same room. I would still be at the school at 11 am. He said fine and hung up. When my husband and I got to the school we were greeted by the principal, teacher and a school adviser. Inside his office was my son sitting at the table. When we got in the office and started the meeting the principal started it with a report that he was reading from and said it was the police report that the officer (BTW is the dare officer for the school) that stated it was not illegal for a teacher to put their hands on a student. my husband and I felt that a "closed door" was already happening and we proceeded to explain that she did not exhaust all areas to try to "defuse" the situation. Like send him out of the room etc. I asked her what had happened and she said she could not talk without counsel present. I then said that the meeting was over and asked what happens to my son and the English class now. He said we will keep him in ISS and that he would work from packets till he could figure something else out. he would call me on Monday morning. My question is What are my rights as a parent and my sons right as a student? Why is it right for him to be isolated for speaking up against a teacher who used unnecessary force on him. Do I have any rights to go further with this matter and find out what exactly happened in the classroom, and if she did hit him was it justified and if not how do I fight to have her removed? Please help. Thank you.