I walk into my bottom set of eleven year olds, and they file quietly into class, sit in silence to do a test that they can barely even read, quietly and politelly raising their hands for help when they get really stuck.

I walk into a top set of thirteen year olds, to cover their absent French teacher, and they ignore me, concentrating instead on racing each to give action lifts so they can smash the ceiling lights in the corridor. It's twenty minutes till they enter the room, despite my exhortations, and the threats of another teacher passing. Once in, they scream swear words at each other, play fight, and race in and out of the room for another twenty minutes. Eventually I get them calm enough to remain seated, and three students do some work. The rest taunt each other, kick chairs, try to secretly listen to ipods or look distinctly depressed by life, as they draw obscene valentine's messages to each other.

I know one of these classes, they're used to my rules and regulations, and know for certain that I will follow through any insurgency.
In the other class only five students have been taught by me before. They know I will follow up infractions of rules, however tedious the process becomes, but still respond absolutely differently to their demeanour when in my room, or following my lesson.

I surely can't have lost all ability to teach in the short walk from my room to the French block.

sure feels like it, though.