I sent the wrong message. 555 people died

The best way is to find him right away, just as a chat, and take the opportunity to say that your mobile phone is dead. If you want to make a phone call, just turn it on. If it's not in the same place, find a good friend, a friend around him, to boot and delete that information. If it wasn't for you, forget the message you sent. If he asks, you can say that your classmate played with it and made a mistake, but he refused to admit it. That's all!