"Remind 101 is a free service that allows you to send text messages to groups of students and or parents from your computer, your iPhone, or your Android phone. The benefit of using Remind 101 over Google Voice, which I used to use for this purpose, to text students and parents is that your phone number is not revealed and your students' cell phone numbers are not revealed to you. Students and parents have to opt-in if they want to be added to your text messaging list. Students and parents have to enter a confirmation code to state that they do want to be contacted by you through the service."
Via John Evans