Blaming unions is really a non sequitur. "Workers organize to ask for better conditions, therefore the quality of the product goes down." No. That was certainly never the case before, and to blame unions for the USA's slippage in educational readiness is just short-sighted ideology.
We're talking about services not goods. It is widely accepted that many service-sector unions, particularly public service-sector unions, have an adverse impact on the quality of service.
Teachers' unions should be pressured to improve. The US pays more than any other OECD nation for K-12 education. If anything, the unions are simply piling more administrators on to the system, while protecting mediocre teachers and failing to attract quality talent into the teaching profession.