There’s a scene in the film Casablanca when Captain Renault asks Humphrey Bogart’s laconic bar owner Rick how he ended up in such a godforsaken place. When Rick replies that he came to Casablanca for the waters, the confused Captain points out that they’re in the middle of the desert, to which Rick responds dryly: “I was misinformed.”

It’s this scene I think of when I consider the economics of supply teaching. Back when I began my journey into the world of the peripatetic pedagogue, registering with teaching agencies, I was promised great daily rates of pay and freedom from the horrors of preparation and marking. All I had to do was sign on the dotted line and soon I’d be earning the same money for half the work. It all sounded too good to be true.

Which, of course, it was.

There’s no doubt that the main benefit of day-to-day supply is no longer having the dark clouds of perpetual assessment and constant surveillance hovering over your head - you go in, you do your job, and you leave at 2.55pm unencumbered by piles of marking or anxieties about the next lesson observation. As for the money… it’s complicated.

The first indication of how much I’d be getting paid came from the agencies themselves. They would quote a typical daily rate and then tell me that with my wealth of experience I’d certainly be able to earn a little more. I imagine the same spiel is given to everyone, the less experienced teachers being told it’s their youth and enthusiasm that will command the extra pay. As with estate agents, be prepared for a certain amount of self-serving bullshit.

Not all supply teaching is equal

There are certainly differences between the rates of pay offered by the different agencies, so it’s worth looking at several. You can then potentially play them off against each other to boost your daily rate. And remember that not all supply work is equal, with agencies paying more for medium- and long-term jobs than for daily supply, which seems reasonable given that this is likely to entail some marking and preparation.

Holiday pay is another vexed issue. The law states that all agency workers in any profession must be paid holiday pay. Some agencies subtract a small percentage, around 12.5 per cent, from the quoted daily rate and hold this back, to be paid at the end of each term. Some merely indicate in your payslip that a percentage of your daily rate counts as holiday pay, which feels a bit like they’re gaming the system. Some simply ignore the issue altogether. Either way, for the new supply teacher it’s important to ask if your daily rate includes statutory holiday pay or not.

New supply teachers should also watch out for the half-day versus hourly pay trick. If you’re offered a morning’s work, most agencies will pay you for half a day. If you work just the afternoon, you’re likely to be offered an hourly rate. The school day is not in two equal halves, mostly it’s four hours in the morning and two after lunch. If you get offered am only, be sure to get an hourly rate or you’ll be doing two-thirds of the day for half a day’s money.

Room for negotiation

Then there’s the delicate art of negotiation. This is where the savvy supply teacher might be able to wring a little more money out of the agencies. The basic economic model is that the agency charges the school a set amount and from that the supply teacher gets the majority with the agency taking a cut. The agencies tend to keep exactly how much their cut is to themselves. The point, however, is that they may be willing to take a slightly smaller cut if they really want you to work. There’s room for negotiation and I’ve been paid rates of between £130 to £150 for daily supply by the same agency.

For the most part, there’s more work than there are teachers, which should mean supply teachers can make the economics work in their favour. There are certain times when negotiation might be more likely to pay dividends. A call that comes through to you later than usual - i.e. after 9am - usually means someone is desperate. The school probably found out late that it needed cover, and most supply staff will already have been assigned a job by this time. There’s a good chance that if you ask for a little more money, you’ll get it. This extra leverage can also work for certain schools. All supply teachers have a mental list of the schools they like to work at and the ones they avoid.

Schools and supply agencies know this, too. It makes sense to ask for more money to work at a school where you know you’re in for a tough day. You can call it danger money.

I now earn less than I did as a full-time permanent member of staff. But I have far less work to do and I definitely feel far less stressed. If I worked every day, I could probably take home about the same as I used to. But I like Fridays off. And I like to leave at 3pm. And I really like having my life back.

The writer has recently taken up supply teaching after 20 years in a full-time teaching job