More than 50,000 teachers decided, last year, to leave the classroom and pursue their careers elsewhere. The government should take note: it has succeeded in making teaching so difficult and so exhausting that it has created the highest flight from the profession of all time. Perhaps someone should award ministers a medal for turning the teacher supply challenge into a crisis?

What do teachers do when they pursue pastures new? Surprisingly, the majority remain in education. Fifteen per cent of teachers who leave the profession before retirement become teaching assistants (TAs).

I think this is very revealing. Teachers are voluntarily choosing to relinquish a higher salary, greater job security and an established career structure, but opting to remain in the classroom working with children and young people. The things that brought people into teaching remain pull factors keeping them in education.

As I have written before we cannot afford, as a country, to be so profligate with our teachers - particularly at a time when pupil numbers are rising so sharply.

But teachers who are thinking of becoming classroom assistants should take note: the grass may not be greener. Reports from ATL’s support staff members show that as school budgets decline in real terms, the pressure on support staff to take on more work, and more challenging work, is becoming greater. In too many schools cover supervisors and learning support assistants are being used to cover classes - with timetables which require them to act as supply teachers - but without the pay and recognition they should receive for this.

A young man I met recently at ATL’s support staff conference wants to train to become a teacher and is working in a school as a curriculum support assistant. Since September he has covered, on average, 13 lessons a week. In that time six teachers have left the school and not been replaced. Their timetables are being shared out among the existing teaching staff in the school, and among the support staff. He told me that he is teaching the pupils. He does not have to plan the lesson, but, as he said: “It’s impossible just to sit there and keep the kids silent. You have to introduce the work, try to get them to engage and become interested in what they have to do, and help those who are struggling. The situation is not good. It’s not fair on the pupils who deserve a qualified teacher, and it is putting enormous strain and stress on me - because I don’t feel I have the knowledge and skills to give the pupils what they need.”

‘Paltry pay’

This young man, it should be noted, takes home about £1,000 a month after tax. This paltry pay in no way reflects the responsibilities he is being given by the school.

This is, unfortunately, not an isolated example of exploitation. My email inbox is filling up with more horror stories as the weeks go by. I recently received an email from a member whose daughter-in-law is a classroom teaching assistant. She wrote: “Her contract is constantly being changed along with her hours and conditions. New contracts are handed out like toffee. I don’t think anyone is taking the situation seriously, and most of these poor TAs are fearful of losing their jobs. My daughter-in-law is a woman who has given her all for 20 years and who loved her job. She is at present looking for other work outside the education sector. (By the way, the other evening she was marking books at 7pm! Her take home pay is £800 per month. For Monday she has a presentation to prepare and an assessment to get ready for a meeting with the head.) This woman gets outstanding at her assessments and outstanding from Ofsted. Just to finish, this week she has covered eight lessons with no one in the room with her. She is not a teacher!”

Let’s not beat about the bush here - these classroom teaching assistants are being exploited. Too many school leaders, increasingly concerned about balancing their school budgets, are misusing their classroom support assistants, and are making unreasonable and unrealistic demands upon them, including making them take on a teaching role in the school. Many classroom assistants have teaching qualifications; others have degrees. They are highly qualified, but they are not employed, nor remunerated as, teachers. They do not want to teach (otherwise they would be training as, or working as, teachers).

I have every sympathy with school leaders who are facing a perfect storm of crises - in teacher recruitment, teacher retention and rapidly increasing pupil numbers. But my sympathy does not extend to school leaders exploiting education professionals by expecting them to undertake work for which they are not paid and which is not in their job description.

And my final point: in the end, stretching staff in this way will affect pupils, particularly those who need extra support so they can make the most of the opportunities offered by school. Teaching assistants who are teaching classes are not available to support teachers, or to give one-to-one or small group support, which is, for the most part, just what they are employed to do.

Dr Mary Bousted is general secretary of the ATL teaching union. She tweets as @MaryBoustedATL

Want to keep up with the latest education news and opinion? Follow TES on Twitter and like TES on Facebook