There are reasons that spending per student on education has increased over the past four decades. There are reasons that the number of staff members per student in education has grown over time even though average performance scores haven't changed much (see this).
One reason, of course, is the growing strength of the teachers' unions. Another, more important reason, is opportunity costs. As technologies improve and as capital grows throughout the economy, labour becomes more productive. In a market economy, this increased productivity means that labour will be paid more and be offered better working conditions.
Even if there have been no technological changes and even if the capital/labour ratio has not changed in education, if workers in other occupations are increasingly receiving better offers over time, then school boards will have to improve their offers for potential teachers if they wish to attract and keep them.
In other words, to hire teachers, the employers must meet or exceed people's opportunity costs. And these costs are constantly rising as potential teachers have better and better options outside teaching.