Remind101 and Celly both offer a way for teachers to keep in touch with students and parents of students by using SMS on US mobile phones. This makes them impossible to use outside the USA, you would think. I haven't found a way to use Remind101, but I think I have for Celly.
These two systems are similar to WhatsApp, but with some important differences. The most important one is that Celly and Remind101 both screen mobile phone numbers, so your students can't phone you and you can't phone them. In many teaching situations this is legally necessary.
There are apps for both systems, but you can't download the ones for Remind101 outside the USA. With Celly, it is much easier for two reasons. One, you can download the apps (at least) in Spain and the other is that you can use Celly in a web browser.
The procedure to follow is explained in this 'training video' I've made for my students:
I've made a much smaller version, but it is still very slow to start.
Basically, you go to https://cel.ly/ and sign up for an account as a teacher and create a cell for each of your classes. Then you ask your students to go there and sign up using their real names, or possibly their first name plus an initial from their surname.
The students can then ask to join the cell you've created for them. Here's one I've created that you can ask to join:
Don't invite students to join your cell. Just tell them the name of the cell they should ask to join. Thay way they won't have to use SMS at all.
I have the Android app working on my Samsung Galaxy S3 and on my HTC Wildfire S, I can use the web browser and log in to https://cel.ly/