Three of those free periods are on a Friday. This means you can be on the chalkface, without a break - barring intervals and lunchtimes - for up to 13 hours of continuous contact with classes aged 11 to 17. Little wonder you need to relax on Friday.
Because the school is big and classrooms are full all the time, you do preparation in the "base", which is approximately 11ft by 17ft and used by 11 people. There are cupboards and shelves down each side of the room.
The gap is filled with a fridge and two filing cabinets at one end and a door at the other. The door gives access to a small bookstore, which houses the department's computing facility (one functioning and one non-functioning computer). The remaining space is home to two tables and seven chairs.
You have rights to one shelf and a filing cabinet drawer. This is the handy place to work, the only alternative being the staffroom, which is two flights of stairs and two lengthy corridors away. But you spend most of your time answering the departmental phone and looking for the principal teacher.
The time that you spend in preparation is usually spent organising your boxes. As a peripatetic teacher you must have a system. The fact that you only ever see three of your classes in the same classroom more than once a week, and that you are only in the same classroom twice a week for two consecutive periods, means that your system is a class box.
The class drop their stuff in it as they leave the room. Without a regular clear-out, these boxes increase in weight - not least if you are carrying up to 30 textbooks.
As a young and fit supply teacher, you don't mind a bit of carrying, but you don't want to have to carry two of these boxes to two successive classes in different rooms of the home economics department which is down 100 yards of corridor and up two flights of stairs.
Thankfully, you are not always out of the department. You get a chance to visit it at least three times a week, as well as nine classrooms in modern languages, social subjects, mathematics, home economics and the decrepit old tech.
When you arrive at a classroom, the pupils have already arrived. They never sit in the same place twice and, because you seldom see them in the same surroundings, you are vague about their names.
Awaiting your arrival, their spirits are already high. With economy in mind, you distribute your materials. With a school population of 1,200 and a department budget of #163;6,000, economy is an important concept.
Last year the department economised by buying lined paper without margins - then realised that the money spent on rulers to draw margins outweighed the original savings. #163;6,000 divided by 1,200 pupils is #163;5. Assuming four periods a week of study and a 38-week year, this amounts to approximately 3.3 pence per pupil per period.
No problem, you think, this should easily cover the cost of some paper, an occasional jotter and even a pen or folder. But hold on a minute.
This money has also to cover photocopying, risographing, audio-visual resources, computers and - perish the thought - books. You never cease to be amazed at how #163;6,000 can be stretched to cover so few books and so many blank pieces of paper.
So you hand out some more blank pieces of paper, because it is a physical impossibility for you to take a television, tape recorder or computer with you to any room. Fortunately you have been given a small Secondary 1 class of 29.
On the other hand, senior management has targeted your second-year class,because of its trouble makers; you have two generalfoundation third years,with behavioural and learning difficulties; a generalfoundation fourth year, and a motley bunch of S6 two-year Higher pupils who are usually too tired to work because they are up at 5am to work in the local supermarket.
In every one of these classes, in spite of the accommodation and lack of resources, you do your utmost to keep control. You shout, harangue, chide, plead and dish out punishment exercises, but problems still have a knack of surfacing.
The kids have as much difficulty adapting to an ever-changing environment as you do. At this stage, the school discipline policy comes into its own.
The headteacher has requested that teachers only refer pupils to management as a last resort.
As a supply teacher, you do not want to be seen taking easy options, so you want to refer your pupils through the correct channels to the assistant principal or principal teacher. As luck would have it, you are at the opposite side of the school and without a telephone.
But the picture painted should not just be grim, and there are some bonuses. There are industrious professionals who are a huge help to the supply teacher, in terms of resourcing and moral support. There are even some light-hearted moments with children and classes. Memorably, one pupil took the initiative to design a T-shirt for you, in the style of a rock group promotion. "Teacher on Tour" it said. These venues were listed below:
"A19, B26, B13, , K2....Sold Out, Sold Out, Sold Out".
So, against all odds, you carry on, working determinedly to pay the bills and keep your head above water. Another session will soon be done, the National Insurance contributions will be paid and, as one tactless principal teacher pointed out, "It's all right for you. You can sign on over the summer".