Teaching is the best job in the world - but that doesn’t mean it isn’t stressful. This is the real reason why schools struggle to find staff. Your non-teacher friends won’t understand this and nor will society. Teachers hear the bell go at 3.30 and think of the holidays. You will know soon. This is how I found out.

Before I started my PGCE, I went to see my inspirational teacher for advice. “You’ll face stress unlike anything you’ve ever experienced,” he said after I had observed one of his lessons.

I laughed in the ridiculous way only the truly ignorant can laugh. He had just made teaching look so easy. I let him know what stress really was. I told him about one day in my life as an engineer - the “flue job”. The flue was a 5ft-high tunnel, stretching over 200 feet along the top of the ovens in the largest brick works in western Europe. It was designed to draw out and recycle the hot air from the ovens, while filtering out the smoke and dust.

My job was to clean out the entire length of the flue and look for holes. I had two days before they switched the ovens back to their usual 1000xC. Armed with only two buckets, one shovel, 150ft of extension lead and five light bulbs, I set to work on the most physically exhausting job of my life. Starting at the open end, wearing a face mask I filled the buckets with years of accumulated rust and brick dust. I carried it all, bucket after bucket, to the end and threw it over the edge into a skip 30ft below.

To make this torture even harder the floor was punctuated by man-sized holes which dropped into the furnace below. These holes seemed to beckon me as I staggered back in the semi-darkness with 15kg of junk in my hands. Two hours later I had removed the overalls and looked like Bruce Willis at the end of Die Hard.

They may have switched off the ovens, but the residual temperature in the flue was still around 40xC. To get a feel for this, go to the Sahara desert and carry two buckets of sand all day, jumping across an imaginary hole every fourth step. Do it for eight hours. I fought fatigue, fear and dehydration.

Later that afternoon, a colleague arrived to see how I was doing. He found the whole flue experience novel and sprang alog to see my progress. Unfortunately, as he reached me, he mistimed a step in the gloom and fell down one of the floor holes - only his protruding elbows stopped him falling a long way into something very hot.

The man screamed in agony, yelling “I’ve broken my leg!”. I wondered how an ambulance would get him out of there, and seriously questioned my choice of profession. Teaching couldn’t be any worse, I felt. So, I told my inspirational teacher, I doubted a room full of lively teenagers for a mere six hours a day would really compare.

“How did your day start?” he asked. “Don’t tell me - with a skive, a look at the paper. What if you were ill or sick or didn’t finish? If you got it wrong, did it matter in the grand scheme of things?” I looked troubled. “If a teacher gets it wrong, it can ruin lives,” said my teacher. “If you are sick one day, your classes can lose the entire thread of a term’s work. You cannot take a day off when you feel like it.

“Your flue was an adventure. It’s not real life. If you still doubt me, ask the new physics teacher next door who arrived last year from industry and who is going back there as fast as he can.”

Five years later, I have to agree. I never had sleepless nights over the flue or any job. I went home and forgot about it. Now I lie awake wondering how I could change my pupils’ attitudes, or fret about a student’s family life.

If you work in an office, or even as an engineer, you can ease into the day if you want to. In an office, you will arrive at work, maybe get a coffee, have a chat, check e-mails. A teacher has to be alert from the moment they arrive in the building and, if you are feeling under the weather, you have to work doubly hard. Your lunch hour is not your own, nor are your breaks and evenings. How does coursework marking take up so much time?

The worse thing is you can’t just turn up at work and get by. You have to prepare. So if you do have a social life and skip preparation, your day at school can go very badly. No one understands this until they do it.

Remember, though, the rewards are higher. You will make a difference. You must learn to let go, switch off, keep hold of your life. Do that and you will still love this job 20 years from now.

Ray Dexter teaches chemistry at Caterham School, Surrey