I have been using Remind 101 for over a year now. It has changed how I communicate with my students, and I have nearly 100% connection rate...meaning more of my students get my messages now than before when I used to use email or post something on the class website.
Here is a screenshot of one of my messages I sent after I learned that some of my students would be gone for math league.
I especially like Remind 101 for communicating with my National Honor Society students that I advise. I don't see most of them in class, so sending a text reminder is so easy and convenient.
How it works:
Once a teacher sets up an account at Remind 101 the teacher enters his or her classes and each class is given a code. Students text the code to a specific number, and an account is created. There are even printable handouts that explain this if you need! Teachers visit Remind 101 or use an app (See the iPhone app here) to send text reminders. A log of your reminders is kept for you, along with a list of the students on each list. So nice!
I know there are other tools that will accomplish the same goal, so use whatever works for you. Remind 101 has worked well for me and I recommend it! I also recommend adding yourself as a student so you receive the texts as well.