Remind101 is a very easy to use text messaging notification system. You can send a text message to every student who subscribes, and they cannot send messages back to you. You can also set messages to go out at a specific time, so if you are busy at a conference or in a meeting when a message needs to be sent, you're all set!
Pros: This is an alternative to contacting students via email, which they seem to be checking less frequently. Students are not able to text you back so they cannot flood your phone with text messages.
Cons: You cannot force students to subscribe, so this system doesn't work for every student, it is just an additional way to disseminate information. Also, students cannot text you back. You may not like this feature - if so, you should use the Google voice app to text.