Reading their FAQ section reveals that there are some thoughtful restrictions in place. Only the teacher can send a message to the group. If a student replies, then the teacher is able to have an individual conversation. The teacher can not initiate a private conversation themselves. (I am not sure how this is controlled; maybe a timeout period where the text and reply text are no longer actionable? I have reached out the them and will let you know what they say.)
Lets take a look at the signup, layout, and features with a full screen by screen review.
1) Email and password
2) Add your name (it will be used in some of the messages), and provide the course's name
3) Setup is complete! A teacher could print this page to post it in the classroom as instructions on how to sign-up. The phone number student's will text the class code to is on it.
4) Here you see me subscribing to the class updates. ClassPager asks for my name (I used Jimmy) and confirms I am now setup and that Dain is my teacher. To unsubscribe at anytime a student can text "stop" to the number.
5) The teacher will now see the student on the dashboard and has the ability to modify the name and remove them. Notice that the student's phone number is not shown. (An email was not dispatched to the teacher, me, when Jimmy signed up. It would be a really good addition to the service.)
6) Text messages up to 140 characters can be broadcast to the classroom.
7) Sending a message
8) The message is saved on the screen
9) The student receives the text
10) The student can respond11) The teacher sees the reply in the dashboard. (Again, an email notification would be excellent.) Since the teacher is not on the monthly plan they are not able to reply. This is OK as long as the teacher explains they are not able to individually reply. Otherwise it would be nice if ClassPager could send back a text message saying the teacher can read, but not reply. It would be bad if a student thought the teacher was ignoring them.
Teachers get one classroom for free with unlimited messaging. For $9.99 a month they are able to add five classes, also with unlimited messages, and the feature to reply to individual students is activated. There is no fee for students to use this beyond their regular carrier fees. Twilio's SMS API is used to power the messaging capabilities. Keep up with ClassPager's progress on Twitter; they have plans beyond the individual classroom and are are currently testing school and district level programs.