This is the true story of a day in the life of a 17-year-old boy, Shaun, who worked for me briefly before he got into a drunken, violent fight and was sent to jail. His background is one of poverty, chaos and lovelessness.

On the day of his release, he is luckier than most: he has somewhere to go - his girlfriend's bedsit. But Shaun comes complete with tagging machine. He wears a grey tag on his leg and he has a 6pm curfew. Every evening, the machine and tag must renew their acquaintance or Shaun will be sent straight back to jail.

Three days after his release, Shaun and his girlfriend have an argument. She chucks him out at 10am. Now the clock has started ticking because he has to be installed somewhere suitable - a friend's sofa will not do - before curfew. Shaun grabs a newspaper to look through the accommodation ads on his way to his probation officer, who needs to approve the move and must urgently arrange for the tagging machine to be installed in Shaun's new accommodation.

At 11am, Shaun is told that the officer is out. But his journey hasn't been wasted - someone agrees to read the small ads to him, because Shaun can't read. They even let him use the phone as he doesn't have a phone of his own.

It's now noon, and Shaun is still frantically phoning landlords. He's an ex-offender on a tag and most people don't want that kind of tenant, but at 12.30 Shaun finds a bedsit. The landlord is well-known for his low rents and high bacteria count, but Shaun just needs an address by tonight. The landlord wants to meet him at the bedsit. He jumps on a bus and crosses town. The bedsit is not attractive. In fact, only a desperate man would take it. Shaun is a kid, not a man - but he is desperate. He agrees to organise references and return shortly with two weeks' rent and the deposit.

He needs rent, deposit and the right housing forms from the benefits office for his new landlord to sign. It's 1.30 and there's a queue. Shaun bites his nails until it's his turn. A woman says sorry, she needs 24 hours' notice to give him the deposit.

At 2pm, Shaun phones me. I am out of town, but I arrange for a mate who owes me money to hand over all the cash he has, which will cover the deposit. Shaun asks the benefits office for a crisis loan for the two weeks' rent. They tell him to come back after 3pm. I phone through a reference to his landlord, while Shaun dashes off to meet my mate, and by now he has no money left for the bus so he runs. He's running against the clock.

His landlord takes the deposit and signs the forms. Now it's 4pm, so his probation officer must be back. Shaun rushes across town and breathlessly explains what has happened. The probation officer has a heavy caseload and is not happy, but he agrees to endorse Shaun's new address and rings the landlord with the necessary second reference. He contacts the tagging team and asks them to install the machine at the bedsit before six o'clock.

Shaun just makes it back to benefits with the signed forms before they close. And, yes, they've got two weeks' rent for him. If he can give this to his landlord and get back to his new home by 6pm, and if the tagging machine is in place, Shaun will be able to stay there. With not a moment to lose, he makes a last superhuman effort, running all the way.

It is 5.45. Shaun staggers through the door. The tagging machine is there and so is Shaun. He's done it! Except he hasn't. The machine runs on electricity and the meter's empty. Shaun needs a £1 coin. And he doesn't have one.

Shaun is just a kid. He sits down and cries, and then he rings me and cries some more. I arrange for someone to cross town to deliver some coins. But it is too late. By the time my mate arrives, Shaun is deemed to have broken his curfew, so it's back to jail.

Most kids his age have spent the day in college, where they probably achieved little compared to what Shaun has. While they face career choices, Shaun today learned that, for him, change is obstructed and his choices are few. No matter how hard he tries.

• Mark Johnson, a rehabilitated offender and former drug user, now runs a charity that aims to reduce reoffending.