You can't always send text messages to your classmates!

First, where are you? Are you on the train and entering the tunnel? The signal there is not good, because I can reply to my friends in 20 minutes after I go home. Second, make sure your friend's cell phone is turned on. If his cell phone is turned off, you can't send it, and you can't send it until he turns it on. Third, make sure you have enough mobile phones. That's about it.