It's worth pointing out that unions operate in the private sector as well (and compete against non-union firms).
Unions in the private sector operate in a fundamentally different sort of "reality". That's why even some historical supporters of private sector unions used to say public employees shouldn't be unionized.
Private sector employers take their profits to the bank, or take on debt for themselves. Public sector employers take the savings to the public treasury. lol. And we pay the taxes they need to run their programs. Public sector employers represent the "real" owners, the "people". The pernicious evil that results from public sector unions is a doubled-down clique of folks who whine and dine and create their own little clique of "insiders" whose existence is wholly at the public expense,and against the public interest.
To prevent this, and yet still promote the cause of good public education with the better teachers on the job, the public would be better served with an independent performance commission that operates like a check and balance on the public management, and a performance review function that looks at teacher merits/dismissals. Not a State or Federal oversight body like a state "board of education" or a Federal Dept. of Education which in effect guarantee the development of "insider"/"elite" management which is not responsive to parents and acts at every decision to raise costs of education without concern for the interests of the students or their parents, but instead promote global or national propaganda/indoctrination objectives. Unions and the public education establishment, for whatever "good" intentions that some might have, are an overlord form of governance that is inherently against the public interest.
Management that is hobbled from making essential decisions to fire poor teachers by a maze of rules and procedures resulting not from parent/management meetings but union/management backroom deals, or is in any other way diverted from making the necessary and right decisions, should not be tolerated. When parents/students are being cut out of the deal, we should fire them all.
Vouchers would be a minimal step in the right direction, allowing the parents/students with the guts to make their own decisions the power to do so, with their own tax money.