Imagine it’s your first proper lesson. It has to go well. You worry you’ll be scarred for life if it doesn’t. So you’ve planned and agonised over acetates, dithered over delivery, and decided that your bum does look big in your new trouser suit. Finally, you’re ready (you hope). It’s 9am, time for the first lesson of the rest of your life.

By 9.20, you’re in full flow. Your punchy opening grabbed their interest. Your pitch was spot on. Now you’re gliding through your audio-visuals with the winsome grace of a weather girl. As you remember their names and ask questions, you think you’re the best teacher in the stratosphere. Now you’re going to deconstruct Hamlet’s language. You fix the class with an intense stare: “Right then. Let us consider the character of this complex individual.” (God you’re good!) “What do you think Hamlet meant by his last words: “The rest is silence? Hmmm?” You pause for dramatic effect. Then a mobile phone starts to bleep. While you try to recapture your lost chain of thought, every student in the class rummages in pockets and bags. Before they were focused, now they’re wriggling in their seats. You have changed from Impressive Teacher into Flailing Failure in nanoseconds.

The lesson is trashed. All you can hear are giggles and a tacky rendition of “Greensleeves”. Forget what they told you about disruptive pupils - the real threat to discipline is the mobile phone.

My response is to paste a large sign on my classroom door requesting that mobile phones be switched off before entering. The day after the warning goes up, a pulse trills from a girl’s bag. I snarl. “It’s okay,” she reassures me. “It’s my pager, not my mobile!” The class laughs uneasily. I winc. What other digital demons have I left out? This may cause amusement, but it is a waste of teaching time.

These communication aids actually achieve the opposite. They not only distract but initiate battles of one-upmanship and anti-social behaviour. There we are discussing poetry when, suddenly, the affable young chap who’s just been so articulate on symbolism has turned his back and is grunting into a plastic box. Worse is the bizarre status students attach to mobiles. Students even assert their group identity through a chosen make and model. For example, there’s a chrome Samsung number which is so expensive that any student who owns one is sending out a clear signal to their peers.

The student mobile phone user is a sign of changing times. I don’t understand voice dialling, or press-on covers, and most of these little gimmicks are harmless. But there are some altogether more sinister aspects to mobile phone use that are only just being uncovered.

Text message bullying is a problem. So is cheating. Now that the latest phones can access the Internet, students can search for answers. Perhaps we will have to frisk tomorrow’s students for gadgets before they sit their exams. Or use an airport-style sensor at the exam hall door.

In the absence of any solution in the battle to stop students using mobiles in the classroom, I suggest the following: lay down the law from day one. Get your students into the habit of switching off phones, pagers and car alarms before they enter the classroom. You could even have a fish tank of piranhas on your desk, with some fluorescent-fronted phones at the bottom as thought-provoking decoration.

As for me, I’ve taken the ultimate precaution - lining my classroom walls with lead.

Cassandra Hilland teaches at a sixth form college in Surrey