There's an interesting discussion of teachers' salaries here. It's a long read, but it also points out that some teachers are "overpaid" and some are "underpaid", and tells why.
I was unable to find the report referenced in the article comparing the salaries of teachers to those of average state workers (although I found a number of references to it), but maybe someone else will have better luck than I.
Your link does raise the question of how to define "average" compensation. One problem is that the changing nature of the school systems means that the workforce in one year may not be comparable to that of a few years later.
The study completely ignores key factors in attracting teachers - or any employee for that matter.
For example, working conditions. When working conditions are such that they have a negative impact on the ability to hire or retain teachers, those conditions need to be corrected. This report says working conditions do not need to be called into question as a factor in teacher retention.
That is a glaring omission.
It is interesting that someone would consider this a relevant point. Rightfully, teacher's salaries should be compared to the results they produce, not how their salary stacks up to other unrelated fields, other government employees.
In the private sector - an employee's worth is determined by their results. No company pays an employee more than that employees output is worth to the company (simple economics).
Granted, schools don't measure success with profitability, so we can not use that gage. What the schools are measured by is the quality of the students that are in the school system and graduating from it.
In a real world comparison, then, teacher pay should be tied to what the students have learned, how they perform on exit exams.
From the discussions I have seen on FR and the teachers posting to them, I suspect teachers do not really want to be compared to the real world. In that respect, then, the comparison you point to is relevant to the teachers, if not the rest of us.