That is a management problem not documenting the progressive failures of the bad teachers when they are not meeting annual appraisal goals and objectives. Document properly and the union has no leg to stand on defending an employee.
In PA, I believe the administrators. Principals for example are also union, either affiliated or part of psea. So they protect each other. Absent abuse of children, indictable criminal acts, etc. it is very difficult to fire teachers here. They don’t have to perform to any metrics. It’s very difficult to get rid of any teachers that have made it through a few years and almost impossible for new teachers to get jobs without knowing someone for an in.
Part of that is the problem of having elected people as the ultimate boss (i.e. governor, state legislators and senators) and they consistently don't act upon or fail to negotiate contract demands (either refuse demands or give into all demands). Also many places it is illegal for public employees to go on strike, for example in Iowa. Strike=arrested and jail.
No such thing here. They strike all the time. There is some local district or other on strike every single year. They are really only accountable to local school boards. Basically the teachers unions spend a fortune electing people that will back them, mostly Democrats, then they are the ones they negotiate with for terms. Then they use strikes as leverage.
Oh and those same school boards determine school/property tax increases.