Many upper-level classroom teachers are using this tool with students because most of them have cell phones. Down here with the elementary students that I work with, I see this more as a service for parents. (Principals, you could use this with your teachers.)
Here is how it works: After you sign up, you will create a class. That class will have a phone number for parents to send a text to and a specific message that they are to place in the body of the text. After that, they are official signed up to receive your messages!
You can even have multiple groups. You could have one if you are a coach, student council leader, or any other after school activity coordinator. You are allowed up to 10 different groups all from your Remind 101 account.
Lastly, these groups are closed. If a parent tries to text back to your remind 101 group, you will not receive it. This also means that if they attempt to text back, it will not go to any of the other subscribers, nor can anyone see who is subscribed to the group.
These is an app available on iOS and Android devices if you prefer to send your messages from mobile devices. You do not have to use a cell phone. This service will actually work over wifi on your iPad.