Was this investigated by the police? From what you have related, I doubt that there would be sufficient evidence to support a criminal charge of battery or assault. Has the student brought a civil complaint for battery or assault? I would doubt that they would because the student himself may have threatened harm (assault). So, given this, what has the district charged you with? Who is the investigation officer and HR director--and is this the proper role for a HR person? Where are you located in California? It soundls like you teach high school. In my experience the involvement of a lawyer tends to make these school administrators reconsider.