This is a link to a forum I recently contributed to which discussed the use of ICT in classes and how to manage the use of computers in schools. The scenarios were well presented by the students and the discussion was very useful.
Fellow contributor was John Oxley and the forum was run by MIchelle Williams.
Image modified from Zigomitros Athanasios work at Wikimedia Commons
For some reason teachers who ordinarily are comfortable with managing students in their class, find the management of students within a computer area such as a computer lab, extremely demanding. One response to this is that teachers ask for technical staff to block websites citing that the sites are a distraction.
Here is a good response to this issue http://tiny.cc/nTk10.
Within my experience I would say that one of the reasons for students being distracted when in class is due to the trivial and/or unengaging nature of the work that students are set. The cure for this is differentiated learning and the use of powerful questioning techniques. A good reference for this is www.fno.org