Celly is a free service that enables you to create and manage contact groups for text messaging. Celly calls these groups "cells" and you can create as many as you need. You can manage these groups from your phone (Android or iOS) or from your laptop. Likewise, you can send messages from your phone or from your laptop. Like regular text messages you can attach files to your messages.
People can join one your Celly group by sending a text to the join code assigned to your group. People can also join via the web by going to the unique URL assigned to your group.
Applications for Education
From the perspective of a teacher or administrator Celly's big appeal is the option to archive all conversations that occur within a Celly group. You can set permissions in Celly groups to allow replies to messages that you send out. Or if you don't want to receive replies you can set permissions to not allow replies.
For students and parents who don't want to receive text messages, there is an option for them to receive email alerts instead.
Celly wasn't quite as intuitive to set-up as some similar services that I've used. I recommend watching the Celly intro videos to get started.