My wife is a middle school teacher and created a system similar to this using a Gmail account. She creates groups for each class, then emails to their SMS address (phonenumber@att.net or similar). This looks so much nicer than jacking around with all the various carrier email addresses manually. I'll have her pass this along to her district. Best of luck!
Thanks! If she or other teachers have any questions, they're welcome to email me: m [at] classpager.com.
I think the school/district-wide system is going to be your best bet. As I'm sure you're aware, teachers don't exactly bring home the bank. So personally dropping $10/mo for something like this isn't likely going to happen for most teachers (most provide quite a bit of in-classroom supplies out of pocket as well).
Something to consider in promoting to schools/districts: lack of parent involvement is one of the biggest reasons kids aren't doing well in school. By keeping parents informed on a regular basis about what their kids should be doing, there's a better chance homework will get done, studying for tests will happen, and those damn standardized test scores will improve. Well, that's the hope at least.