A quick summary of a conversation I had today about sending text messages to multiple recipients.
Remind101.com is set up more for individual teachers, but it may do what you need.
Twilio is a platform for building sms and phone call apps/applications.
If you know which wireless provider each of them subscribes to, you can
use email to sms, and even set up a Google Spreadsheet to do that.
Or you could build an Android App (using the App Inventor if you need) that sends multiple messages from your smartphone.
Just a few of my thoughts, hopefully that helps.
Edit: I was reminded that Edmodo allows students and parents to subscribe via email or SMS.