Just before half-term I was working in one of the six or seven local middle schools that regularly employ me. I’ve been “supplying” for about seven years now, so I’m well known on the circuit. Things had been a bit quiet, so I was leafing through The TES when I noticed a new agency. I rang it.

The response was professional and brief. Having established that I was a bona fide teacher, the agency arranged to call me later at home for a more detailed interview. This took place that evening with a pleasant Australian woman.

Unfortunately, she didn’t seem to understand much about the state education system in Britain. One of the first items she mentioned was money, presumably because the agency perceived this as a sticking point. It turned out that it would pay me Pounds 73 a day - about Pounds 50 after tax and national insurance contributions.

I am nowhere near the top of my scale, but this salary would have meant a drop in income of about 25 to 30 per cent. This wage is across-the-board, regardless of experience or qualifications. But I kept my anger in check and continued the interview.

The woman then requested the names and phone numbers of two of the schools I attend, at which point I suddenly realised I could be shooting myself in the foot. What if the agency rang up my regular schools and offered to locate and employ supply staff at a cheaper rate than the schools currently paid me through the local education authority? I explained that, in view of my doubts, I would like to withdraw my application.

I want to know who benefits from this additional layer of bureaucracy. Supply staff would feel exploited and abused by this hefty slash in their salary. All the heads I work for operate from a private list of staff taken from the county list. So who benefits?

The head of operations in my county runs a series of checks on potential supply staff - including police and medical checks - and takes up teaching references before admitting anyone to the list. I don’t believe that agencies are following the same rigorous procedure. On the contrary, I know one agency recently welcomed someone who had not entered a classroom for more than 25 years.

I mentioned my experience a few days later to the head of one of my schools. He told me that the agency I had been interviewed by was offering teachers to schools for Pounds 115 a day. The most expensive teacher he employed cost Pounds 110 a day - so he wouldn’t be using its services.

So the trick is to go down the lists of supply teachers and hook the least successful on to your network. You charge the school Pounds 115 a day, pay the teacher Pounds 73 a day and make Pounds 42 a teacher per day from the public purse.

Can this really be good for our schools? It’s certainly not good for me. Who said money doesn’t grow on trees?

Nick Melton lives in Weymouth, Dorset