Utopia, you are standing in it!

Unions fight for better playing conditions for their members. Is not that what the class war is all about? Unions are not there to act as management consultants to the employer, working out ways to make their wage slaves profitable – extract more labour surplus.

Please do not mention that the employer of teachers is the state sector. That is an argument against unions in the state sector, a very slippery slope.

When the interests of the union and it is teacher members and the interests of children conflict, the union will do what its mission is which is to protect its members.

The first jobs to be unionised were craft jobs. The craft unions certainly foster the marketability of their members but were keen to suppress competition nonetheless. To quote Charles Baird

Most unions in the private sector are in crafts and industries that have few companies or that are concentrated in one region of the country. This makes sense. Both factors—few employers and regionally concentrated employers—make organizing easier. Conversely, the large number of employers and the regional dispersion of employers sharply limit unionization in trade, services, and agriculture. A 2002 unionization rate of 37.5 percent in the government sector, more than four times the 8.5 percent rate in the private sector, further demonstrates that unions do best in heavily regulated, monopolistic environments. Even within the private sector, the highest unionization rates (23.8 percent) are in transportation (airlines, railroads, trucking, urban transit, etc.) and public utilities (21.8 percent), two heavily regulated industries.

Craft unions opposed unionisation of less skilled workers because it threatens their own ability to extract higher wages as explained in the Wikipedia entry:

The concept of organizing a strong federation on the basis of craft evolved out of conflict between the Knights of Labor (KOL), which organized mass organizations of unskilled, semiskilled and skilled workers by territory, and the American Federation of Labor (AFL), which organized only skilled workers.[1] The craft workers were capable of demanding more from their employers due to their skills, and therefore organized into stronger organizations pursuing narrower interests.[2] The AFL was formed as a direct result of the perceived need by skilled workers to defend their individual craft organizations from poaching by the Knights of Labor.[3] The Knights of Labor believed that skilled workers should dedicate their greater leverage to benefit all workers.[4] Selig Perlman wrote in 1923 that this resulted in “a clash between the principle of solidarity of labor and that of trade separatism.”[2] The trade unions “declared that their purpose was ‘to protect the skilled trades of America from being reduced to beggary’.”[5]… As long as the craft unions were the dominant power in the AFL, they took every step possible to block the organizing of mass production industries. This led to challenges from both inside and outside the Federation.

The craft unions such as teachers unions have more bargaining power because they are difficult to replace on short notice unlike less skilled workers. In addition, teachers are much more difficult to automate away. I am not too sure what the teachers union might think of giving more responsibility to teachers aides?