One solution is to use Twitter, as it allows students to post questions and comments. If everyone in the class follows everyone else, then when work starts for the class period, everyone opens Twitter and gets to work. While Twitter can work great in high school applications where the expectations are that students will be responsible with their social media usage, and generally students are above the age of 13, thus eliminating worries of COPPA compliance, it may not be the best choice for middle school, or younger classrooms.
Twitter requires an original email account for each Twitter account, and if you are working with younger students, you may/will probably want to maintain control of each account through managing password change requests and being notified of private messages being sent. All of this can be done by setting up a separate ghost email for each student that is controlled by the school (including the school maintaining control of the password to the account so students cannot access it), setting a default password for each student on Twitter, specifying that password change requests and private messages on Twitter require email approval, and then forwarding all the student ghost emails to an admin or teacher controlled email so you can keep track of what students are doing with it. What a pain!
An easier solution would be if you had a school controlled message board where students could just post. (No school controlled message board? Try Twiducate) One of the advantages of Twitter is that it automatically lets you know that you have new messages waiting, and in a traditional web page you would spend all of your time pressing refresh to see if any student had more questions.
The answer is to set up your web browser to automatically refresh a specific web page. Most browser have some type of plug-in that can be used. Ashish Mundhra provides an excellent overview of what tools to use to make your web page autorefresh.
With autorefresh and message board, now you can keep track of who asked what, and when. You can even have the kids set up autorefresh for themselves, so you are constantly able to see what questions their peers have that they can answer, or if anyone has already answered their question.