Some things are great when you split them: bills … bananas … atoms.

Some things are better off whole: dessert … hairs … 9Y3.

Ah yes, 9Y3. That tricky class with an interesting mix of behaviour and attainment.

“It takes a while to get to know them,” their last teacher tells you. “Once you crack them, they’ll be fine. You just need to suss them out.”

But suss them out you can’t, because you see them only two out of four lessons in week one, and then just once in week two.

The rest of the time, they’re being seen by your colleague. A non-specialist drafted in because of staff shortages.

Because this is one of those split classes.

In that joyous period in the summer term, when certain members of staff are locked in rooms and chained to a computer programme, the timetable is created. They mutter about “balancing allocations”, and can be seen chucking paper in the air when yet another update changes the hours assigned to each subject.

“Core PE has one hour. Two. One. We’re giving science seven hours. Six. Top sets seven, bottom set six and an extra hour of maths. With a maths teacher? With a warm body. What about triple science? Start again.”

Starting again isn’t something that happens only before the timetable begins - timetables seem to regenerate across the year with more ferocity than time-travelling doctors. Staff movement can mean that new timetables will be landing in the trays of teachers several times between September and July.

With every change, splits seem to be happening more and more.

Splits: the difference

It’s easy to see why; with less money in the budget, and less flexibility over class size, and fewer teachers due to the ever-increasing shortages, it is inevitable there will be a falling number of classes that are fortunate to have just one teacher.

Some informal surveys of 30 schools informed me that splits are a growing issue, with more and more split classes as fewer specialists are shared out between pupils.

A single teacher per class is always going to be preferable: the teacher knows the class, the class know the teacher; everything is instantly made easier.

But teachers, more often than not, will be forced into splits whether they like it or not. And with the recruitment issues showing no sign of abating, more teachers are going to find themselves tackling a shared class.

And splits almost always involve non-specialists having their arm twisted to take on a class. That creates multiple problems, not least extra workload for the specialist in the pairing.

David Walton, a history teacher in Essex, explains that using non-specialists with split classes has resulted in lower recruitment for GCSE and then A level, “as well as a lack of skills for students who do opt to study the subject”.

This is both disheartening, and unsurprising. Splits solve timetable problems by plugging the gaps, pulling in teachers from other subjects. But having QTS doesn’t necessarily mean that everyone is able to deliver the lesson the same as a teacher who has a degree in their specialism.

Behaviour in these lessons can also become a pressing problem for senior leaders. Students can sense when a teacher is being forced into teaching the subject against their will. And they try and capitalise on that.

In schools where behaviour is generally well-managed, this can be an inconvenience: you’re more likely to get parental complaints, and this spoils the normally good relationships between home and school.

In a school where behaviour is generally a challenge anyway, this is a nightmare: problems in this class can carry over to the next, and then you find that your whole day is disrupted chasing up problems that have started in the original lesson.

Make like a banana and split

However, there are times when a split can be tolerable, even beneficial. With A-level classes, for example, a two-unit course is often divided between two teachers. In these cases, teachers deliver one unit each with minimal crossover. There is no stepping on toes, and no mixed messages.

Students get the benefit of two teachers’ expertise - the difference being that they are usually both subject specialists.

The lesson, then, is that split classes can work where teachers have confidence in their subject knowledge and clear ownership of the unit they are teaching. But where this isn’t the case, leadership need to provide the right levels of support to fill the gaps and prevent the arrangement from becoming painful for everyone involved.

Grainne Hallahan has been teaching English in Essex for 10 years. She tweets @heymrshallahan