GupShup provides you with the ability to make this happen for free. You 'simply' register for free on their site and then start creating groups. The site has quite a few groups there already and some with more than 2,000 members. The groups are split into categories and there's a couple of pages of educational ones, which include quite a few word of the day type services.
Once you register you can either send out messages to the group through the website or by using your phone.
I guess the downside of this is that your students are going to be paying every time they send a message, and I've no idea what the rates are likely to be. I tried to register using my own phone here in Morocco, but never get the confirmation message with my registration code, so that's about as far as I got.
- Anyone else tried this?
- Do you know of other similar services that are free?
To try GupShup go to: http://www.smsgupshup.com