HE is trying to paint you as a "vindictive ex who is using the PO as a way to get a leg up on custody." That doesn't mean the court or anyone else will buy it. Courts see these situations all the time. Those situations DO exist, but there's nothing here to suggest that's the case in your situation. The order prohibits him from seeing her. He's going to her school in an effort to violate the order by seeing her. Even if the prosecutor thinks that's insufficient to prove a violation (which is HIGHLY doubtful), there's clearly a good-faith basis for your report.
So tune out what he's trying to do. You KNOW he will do that. You don't have to give him more power by letting his song-and-dance dictate what YOU will do.