I'd emphasize more that you can use the service without teachers giving the students their personal numbers- I can see it on the first page, but it doesn't jump out at you.

Another idea (and you may already be doing this) would be to log all communications, and maybe even make them visible to everyone in the "class" via a web login. People are less likely to say something stupid if they know it's being recorded, and it gives teachers an easy resource to fall back on if a "he said/she said" scenario comes up with a student.

That being said, I think it's a great way to mitigate risk for the teacher, but I wouldn't let that overshadow the main benefit, which is ease of use.