“Urghhhhh,” go 475 five to 11-year-olds sitting cross-legged on the floor, already wide-eyed and horrified at the thought of no flushing toilets on the Inca trail. “Urghhhhh,” they go again at the thought of scorpions in your sleeping bag and snakes in your shoes - all perils on Clare’s trek, which they’ll be following on maps and a calendar when they sponsor her for Cancer Research UK.

Headteacher Alan Stockley, a council member for the National Association of Head Teachers and a contact of Clare’s since she was a young reporter in the TES newsroom, introduces her as “one of my best friends”. There are two kinds of people in this world, he tells the children: small people and tall people. (Clare is 4ft 11in, he’s 6ft 2in.) And there are people who do nothing and people who try to make a difference. “Clare’s a doer, and that’s why she’s here today.”

Landywood is not in the most prosperous suburb of Walsall, but the families are generous and the school has raised thousands over the years for Barnardo’s, the NSPCC, and a local hospice. It values international links and has partner schools in Majorca, Italy and Denmark - but none yet in South America. And, like many schools, Landywood has had direct experience of cancer - Ken Willis, its caretaker for 11 years, died in March after a 10-month illness.

Clare explains that she’s had cancer twice and been very ill, but she’s “got better twice”. And now she’s in training for her trek to Machu Picchu, the lost city of the Incas. “Who likes PE?” she asks, and lots of hands shoot up. But would they be so keen if they had a strict coach and lessons at lunchtime three days a week? “I’ve been going to the gym since last September and I pretend to climb mountains on those walking machines.”

Clare needs help to unpack her best visual aid, a rucksack full of useful and fascinating objects. Hayley Smith, 10, lifts out a full cold water bottle that doubles as a hot water bottle at night, and a hat with a flap that keeps you warm when it’s cold and cool when it’s hot. “Look what Hayley’s found!” shrieks Clare - that’s the toilet roll moment - but there are ear plugs (if there’s a snorer in the next tent), waterproof trousers (“the size says aged 12 - I’m only small”), and a miniature teddy bear called Little Bear Arthur, with his very own rucksack and a jar of honey.

“Ahhhhh,” go the children.

So what happens when Clare gets back from her amazing adventure and the children have raised lots of money? Mr Stockley asks if they’d like to see her again, hear her stories and give her a big cheque? “Yesssss,” comes the raucous response.

Sarah Bayliss is editor of Friday magazine