Conventional wisdom once dictated that students should try to focus their attention on the specific task at hand and avoid distractions. That idea seems to have given way to the belief that students can carry out multiple tasks simultaneously. New technologies make it easy to access different sources of information quickly and simultaneously. Students seem to multi-task--to work on a class assignment, communicate with friends and attend to whatever else on screen might attract their attention. Consequently, many assume that students raised on computers are adept at multi-tasking. Research shows otherwise. In fact the idea of multi-tasking in which a person does multiple tasks simultaneously is questionable. Essentially what may look like multi-tasking actually involves task switching. People switch from one task to another and then back; they do not do both at the same time. Task switching taxes ones working memory. People can hold only a limited amount of information in working memory, so as one switches attention to a second task one loses track of the first task. Literally, you cannot keep both tasks in active awareness at the same time. The switch back involves an effort to find ones place (i.e., where was I; what was I saying; what was I thinking, etc.). This is exacerbated if one of the tasks is unfamiliar or complex--often the case in school. In essence, thinking deeply about something complex is hard enough without intermittent interruptions and distractions. As cognitive psychologist, Dan Willingham, concludes, "do one serious task at a time" (see Willigham's Multi-tasking video).
For an entertaining perspective on multi-tasking and cognitive overload, see author, Nicholas Carr's, presentation at the 2011 Economist Conference, http://www.nicholasgcarr.com/