Apparently it was believed by the judge that there was no evidence.Okay, well go ahead and try. It's not going to happen.
The school didn't charge your son with trespassing, arrest him, or put him on trial. They may have accused him, but they didn't charge him. They don't have that power. The police conducted an investigation and apparently it was believed that there was enough evidence against him.
But go ahead and give it shot. Call your lawyer in the morning. Good luck.
Look up the constitutional/civil rights of students and the school's responsibility concerning such. I didn't come here for uninformed opinions.