By day Mark Cann teaches politics at The Blue School in Wells, Somerset. But by night - and in his holidays - he is a key organiser of Glastonbury, the largest greenfield music and performing arts festival in the world.

After 27 years’ involvement, the 57-year-old is deputy to Michael Eavis, the Somerset farmer and founder of the festival, and lives most of the year in a flat at the farmhouse.

He became an organiser for Glastonbury in 1981, when it was the first Campaign for Nuclear Disarmament (CND) festival.

“Then about 30,000 people came,” he says. “Now there are more than 175,000. The sheer size of the festival means it’s had to move into a new gear in terms of management and security, but it’s still retained its spirit and soul. It’s such an eclectic event - there’s nothing like it.”

Mark’s role has varied over the years. He was integral in co-ordinating the festival’s licence renewal in 2001 and is now part of a team that monitors the event.

But he remains modest about his contribution. “Clearly, my day-to-day involvement is limited because my teaching comes first - it’s just something I enjoy doing in my spare time,” he says. “Glastonbury is a fact in my life, just as rugby or cooking may be in other teachers’ lives.”

Glastonbury is also a fact of life for a fair share of other teachers at The Blue School - about a quarter go most years. Some go for the whole weekend, but the majority take advantage of the free local resident Sunday passes. So many pupils attend as well that the school always closes on the Friday of the festival (June 27 this year) and has a compulsory staff training day instead.

Jenny Purrett, the head of art, started taking a group of sixth formers two years ago. They collected and stored hundreds of discarded wellington boots from the previous year before spending two days prior to the festival building a “welly-man” sculpture in the Green Fields. Last year, they built a white elephant from old milk cartons. Both went back to the school after the event.

“We are not doing it this year because a colleague, Beth Williams, an important part of the team, has just had a baby, but next year we plan to make a structure out of abandoned tents,” says Jenny.

“At first it seems impossible, but through teamwork and dedication, we make it happen. It’s incredible feeling the atmosphere build beforehand as all around you turns from green fields into this miniature city.”

Robert Johnston, 30, didn’t enter teaching because the holidays coincide with the summer festivals - but it helps. The design and technology PGCE student at Brampton Manor School in Newham, east London, first discovered The Big Chill in 2001 through a festival music CD. He and a friend promptly jumped in a car and drove for eight hours from Glasgow to the festival in Bournemouth. He’s been hooked ever since. “It’s become much more than a festival to me - it’s the focal point of my year,” he says.

Robert met his girlfriend at The Big Chill, and has made a large network of friends through its online forum. “Initially, I was attracted by the music, but now I really love the community atmosphere and the beautiful countryside as well,” he says. “When I pitch my tent, it feels like I’ve come home.”

Julien Carrera, 36, is more of a Reading Festival man (August 22-24). He was in the music industry before becoming a geography teacher at Watford Boys’ Grammar School two years ago, and has been going to festivals every year since he was 17.

“I’ve got a soft spot for Reading,” he says. “It’s a rite of passage for any music lover. A lot of my pupils go as well, and I empathise and understand the excitement they feel for it - it’s about feeling grown up and experiencing some sort of forbidden fruit, while all the time ticking off some 30 or 40 great bands. This year I’ll be going for Rage Against the Machine.”

For more details of festivals taking place this summer, visit www.virtualfestivals.com.