There has been no weakening of the Middle Class as far as compensation has been concerned. The only difference is that a much larger part of that compensation goes to benefits now than before.
And I posted the numbers not long ago. Teaching pays more than most middle class jobs.
These are government union employees, not private sector. When an employee and employer decide how much to pay the employee, the employer will decide how much he is willing to offer based on how much revenue he expects to the employee to make him. If the number is $20 dollars an hour, the employer may then decide that he is willing to pay the employee no more than $19.95 an hour, as any more than that would likely be unprofitable. None the less, he would like to pay the employee the least amount possible in order to make the greatest profit. The employee will also have a number in mind, and would generally like to make the most money possible. Eventually, if the potential employee comes to an agreement with the employer, they will agree to whatever number that both expect to be mutally beneficial.
The employer bases how much he pays someone based on what he expects to be profitable. Some people believe that the employee will be exploited by the employer and paid below his potential value, and thus the employees need to form unions to protect themselves from "exploitation".
However, this does not apply to government. The government has no profit motive and no need to increase profits. There is no incentive for the government to under pay their employees. The employees, however, still want to make the most possible in most cases. So there is still upward pressure on wages from the employees, but no downward pressure on wages from the employer. That explains why government workers are generally paid so much more than their private counterparts.
There is no need for government employees to unionize. All they are doing is leeching of the rest of the middle class.