Fuck public schools, for real
Be civil with the guy. Show him the texts that you mentioned, and in the next breath say that she will no longer have her cell phone at school. Say that this and her being grounded is you doing your part as a responsible guardian. Also say you will not tolerate her being attacked or harassed in any way and will take legal steps if it does not end. If this was a 15 year old boy I'd say give him a good wooping and forget about it, but things aren't the same when its between girls.
Also, record the conversation. If he steps out of line you will have something to hand to the school board, and it will keep you in control of yourself. And yes, it would be legal:
"Ohio's wiretapping law is a "one-party consent" law. Ohio law makes it a crime to intercept or record any "wire, oral, or electronic communication" unless one party to the conversation consents. Ohio Rev. Code § 2933.52. Thus, if you operate in Ohio, you may record a conversation or phone call if you are a party to the conversation or you get permission from one party to the conversation in advance."
http://www.citmedialaw.org/legal-guide/ohio/ohio-recording-law
Good luck. I hate the idea of having kids and having to go through this shit myself, but I know it will happen sooner or later.