The new supply teacher fluffs his Latin lines

Boris Johnson takes a day off from being Mayor to encourage others to do the same

He came, he saw, he got told off for not paying attention in class and then he was heckled by binmen. It was all in a morning's work for the supply teacher at St Saviour's and St Olave's Church of England secondary girls' school – or, as he is more commonly known, the Mayor of London.

The classroom full of 15-year-old girls in south-east London was far from the one at Eton where Boris Johnson conjugated his first ancient verb. But for Boris, there is no fear: he began his lesson by telling the girls about the proclivities of Roman women, in particular their fondness for gladiators.

Everyone was a little awkward. Then in an episode of cunning, he conjured two sentences that he helped the class put together in Latin: the woman loves the gladiator, but the women do not love the charioteer.

The Mayor, former King's scholar (one of Eton's highest awards) and Brackenbury scholar (Oxford) was playing teacher to promote a scheme which aims to persuade companies to give employees a day off each year to be spent helping the local community.

Mr Johnson came to offer his skills as a classicist, and all-round good egg, to pupils studying for Latin GCSEs. Although the subject is not on the syllabus, it is taught in lunchtimes and after hours by English teacher, Sophie Hollender, and voluntary emissaries from Westminster College.

The Mayor's long-lasting affection for Latin comes from his belief in its benefits beyond the realm of dusty academia. "I won't say it's the route to colossal riches," he told the class, "but I read almost nothing but Latin and Greek for 25 years, and I'm now in charge of every bus in London."

He added: "It helps you be more logical. It gives you an understanding of your own language too." There was a ripple of nervous laughter from an audience amused and slightly wary of Mr Johnson, whose bike, bray and bouffant thatch were novel to the surroundings.

He found himself rapped on the knuckles for not paying attention during the class discussion following a clip from Ben-Hur. "That was a bad moment," he confided after the bell had rung. "I forgot I was supposed to be writing down my thoughts and feelings. And when she [the teacher] got to me, I had not a single adjective written on my paper."

He appeared to have quite a freestyle approach when it came to his turn in front of the whiteboard, muttering "teaching is hard", before leading the assembled in a hearty chant of "amo, amas, amat" and a further, rather less certain version of the passive.

So far, so Cambridge Latin Course: the comforting repetition is the same regardless of student or social strata. I learnt Latin this way, studying in lunchtimes and evenings, because it was not on the curriculum at my comprehensive. Thanks to two teachers, one of whom called in a favour from her alma mater Cheltenham Ladies' College (which was throwing out old textbooks), I got a little of what some call a "classical education".

"Maintained schools haven't had enough government encouragement," Mr Johnson said at the end, adding: "I was drained by that. And the kids knew far more than I thought they would."

After answering binmen's questions on the congestion charge at the school gates, he was ushered away for the next mayoral event, wearily getting on to his bike with the admission: "I'm also deeply hungover."