It is final period and I am standing on a chair in the corner of the classroom, pulling a sinister face and flapping my arms.
The Year 11 pupils break into fits of giggles. Thankfully, two students swiftly deduce the poem I am struggling to mime: "It's 'The Vultures' by Chinua Achebe!"
With only a few lessons to go before their GCSEs, their teacher Margaret England has decided that a game of charades is in order to help them to revise the AQA exam board's poetry anthology.
It was one of many imaginative classes I was lucky to witness during two weeks working alongside the teachers in Lilian Baylis's English department.
Ostensibly, I was there as a teaching assistant, though I am not sure how useful I was. Secretly, I thought it was going to be a bit of a holiday - how tiring could it be to help out in a school, especially when you can knock off at 3.15pm and don't have to worry about marking or lesson planning?
I returned home most days feeling exhausted. The combination of early starts, moving from class to class according to a strict time-table and constant vigilance to ensure pupils were on-task was far more tiring than I had expected.
In several lessons, I sat with pupils who spoke English as a second language, helping them to keep up.
Some of the Somali refugee students would amaze me with the speed with which they had picked up the language after only a brief period in the UK.
With others, it could take nearly an hour to help them to label a few simple pictures.
Ellena Valizadeh, a talented newly qualified teacher, neatly summed up the struggle to prepare these students for national tests. "Imagine you'd arrived in Spain speaking no Spanish and two years later had to do a written exam on (the poet) Lorca," she said.
Her fellow NQT, Emily Mills, was equally impressive, livening up lessons with amusing cartoon characters she had created on her interactive whiteboard.
It was also a delight to work with Iain Sankey, the benevolent head of English who buys chocolate eclairs for his department's meetings. As we invigilated a Year 7 exam together, he whispered: "It's like watching paint dry, isn't it?"
But the teacher who kept me busiest was Phyllis Gregory, an award-winning advanced skills teacher who advises the Department for Education and Skills' innovation unit.
She hopes to introduce a documentary video-making qualification, so she put me in charge of groups of Year 9 pupils whom I cautiously led out on to the streets of London. Their behaviour was generally better than I had expected. Yes, some were disruptive, and I found myself having to cajole them or threaten to take them back to their teachers to stop them play-fighting and listening to MP3s on their mobile phones.
"Shouting 'Bastard!' at passersby is not the best way to get interviews," I explained to one pupil, through gritted teeth.
But even the most troublesome students surprised me with their dedication to the documentaries. One of the best teams investigated fast food and quizzed the owner of a fried-chicken restaurant. When he confessed on camera that he only changed the oil in his fryers twice a week, the pupils found it hard to disguise their glee. And so did I.