He and two colleagues founded Central Supply Teachers in Birmingham 10 years ago to offer top-quality teachers at a moment's notice.
Head of St Bernadette's primary school for 18 years, he says the quality of supply staff has dropped.
"A lot of supply teachers have gone into permanent positions and the ones who are left are often the sort who come in at nine, leave a three and fail to mark even the work that they themselves have set."
Mr McNally believes the Government desperately needs to address the issue of retention as well as recruitment if it is to solve the problem.
He said: "What is happening is that young teachers are coming into the profession, finding the pressure too much and quitting after just a few years. If the Government doesn't do something to make the job more attractive, it is going to find itself with a real catastrophe on its hands."