Soldiers swarm the campus.
They arrived in the dead of night, in stealth choppers, making their way almost undetected toward the school.
Almost.
The X-Men met them about halfway, and the first indication the students at large get is a telepathic alert that wakes them all, and then the sound of gunfire.
Students are urged to seek safety, usual protocols being to head to the Danger Room. Some soldiers likely made it into the school itself, but most were driven off before they could get there.
Feel free to react to it in after-the-fact threads, or during-the-fact threads. There were likely some injuries, but no casualties in the incident.