Beware of correlation/causation. Unions that represent the highest earning workers are the strongest, because the strongest workers already have the highest bargaining power and the highest salaries.
A modern example of what productivity does to working conditions is in SV, where the unionized workers have less perks than the un-unionized ones.
Historically unions are always tied with: import restrictions, protection from competition, minimum wage laws (that dont apply to themselves), unemployment (the best way to increase wages for a union is to cut supply short).
As a nice example, Argentina has unions with constitutional level protections since the 50's, and unemployment is up, working conditions are down, poverty is up, import restrictions are up and minimum wage laws are up: result, 40% of the population works under the table, and union workers have some of the highest wages in the country.
You will seldom find improving working conditions without notable increase in GDP per capita.
Charter school teachers are non-unionized, so this demand harms the pay of non-unionized "outsiders". If they succeed, they can parade the statistic that "Unionized teachers have gained income over non-unionized teachers thanks to the wonderful work of unions", when in fact what they have really done is transfer wealth from outsiders to insiders.
Those gains so-called 'worker rights' were achieved in countries without unions as well. It's also unmentioned how much unemployment and misery unions also caused in this narrative.