Perhaps it is the yawning or the tired eyes. Perhaps it is their conversation about "killstreaks" and "frags". Either way, teachers can often spot the signs that a pupil has sat up too late playing the latest Call of Duty. The time children now spend on video games is concerning for many teachers. Those working in primary schools can be downright horrified.
Since their infancy, video games have had an uneasy relationship with schools. To many parents and teachers, they are the arch-enemy of learning. Yet some tech-evangelists have proclaimed them to be the future of education. Over the past three decades, a succession of well-meaning educators and programmers have tried to recreate their magical hold over pupils to improve learning.
However, nearly all attempts to create a great mainstream "educational game" have failed - not because it is an oxymoron but because it is very hard to do both aspects well. Such projects usually end up as either educational programmes with unsatisfactory game elements tacked on, or vice versa.
That hasn't stopped teachers making excellent use of games that do have a heavy educational slant, both in secondaries and, increasingly, on interactive whiteboards in primaries. What may be more interesting, however, is how teachers use mainstream games as inspiration. Witness the work of Tim Rylands, who discovered as a primary teacher that the game Myst could provoke creative writing.
And now we have Jane McGonigal's book Reality is Broken (see extract on pages 4-7). It makes a strong case that the problem society faces is not that millions of people want to spend time playing video games, but that we have not properly considered how we can make the rest of life as engaging as those games, including school.
This is not a call to replace lessons with computer games; it is a call to consider how education could be structured to be similarly satisfying. It is also not a call to replace teaching with easy "edutainment". Gamers don't want their experience to be easy or passive, and they are willing to practise hard to succeed. It is time to harness that engagement for learning.