Remind101.com which lets teachers text (without ever knowing the kids' phone numbers) reminders for assignments, tests, etc. I always set it at the time I make the assignment as you can set the time and date. But now if you forget to do this and you have an iPhone, you can send the reminder on the go. Best of all this service is free! If you want to follow their updates, go to their blog. Their co-founder tells me that the Android app is coming this summer.