Ask any parent, principal, or student who the best and worst teachers are in a given school, and there’s usually a quick consensus. Yet how schools evaluate teachers has been a heated policy debate for decades. It’s one of the main sticking points for the Chicago Teachers Union, now into Day Six of a strike that has left 350,000 children without teachers.

The system of teacher evaluation used in many school districts relies heavily on classroom observations. It rarely counts student achievement on state tests as part of a teacher’s overall evaluation. This model, as the New York Times editorial board notes, consists of “cursory classroom visits by principals who declare nearly every teacher good, or at least competent, even in failing schools where few if any children meet basic educational standards.”

This is an incredibly unfair system for children, who, year after year, are needlessly subjected to ineffective teachers, causing them to lose years of potential learning gains. As noted by Eric Hanushek, one of the first researchers to measure teacher effectiveness on student achievement gains, “A good teacher can get 1.5 years of learning growth; a bad teacher gets 0.5 years of learning growth. If you get a few bad teachers in a row, a student’s life is altered dramatically.”

Moreover, it’s incredibly unfair for teachers. The existing system, effectively blind to teacher performance, compensates teachers based on years in the classroom, not effectiveness. The best teachers aren’t noticed; the worst are protected.

Yet the unions, who claim to represent the interests of both students and teachers, are using Chicago children as political pawns in their quest to fortify the status quo and ensure nothing threatens their entrenched power.

The union was offered (in the most recent framework, currently under consideration) a 10 percent pay raise over the next four years, continued “step increases” based not on merit but on time served in the system, more than 500 new art and P.E. teachers hired by the district, and the creation of a “hiring pool” where any laid-off teacher would have first dibs on any job openings within the district.

They rejected that offer. Meanwhile, 350,000 schoolchildren wait, idle. The main sticking points: teacher evaluations, the length of the school day, and what to do about under-performing teachers.

Those picketing in the streets in Chicago — the teachers, who earn on average, $76,000 per year, and after 30 years in the classroom, receive an annual payment of $77,400 for life, courtesy of the Illinois taxpayer — want to continue receiving this compensation. Moreover, they want it without the accountability that is so badly needed.

That level of compensation, in a district that faces a $1 billion deficit, certainly merits accountability. Teacher evaluations should be based in part on student performance on tests, using a value-added model that takes extraneous factors such as socioeconomic status into account. Those models should be based on growth: Has a teacher imparted a year’s worth on learning in a year’s time, or better yet, a year-and-a-half’s worth of learning in a year’s time? Other factors, such as principal and parent evaluations and observations, should also play a part.

Effective teachers should be well-compensated; ineffective teachers should find another profession. The Chicago Teachers Union will continue to oppose any such reform, but can hopefully reach an agreement over the next 48 hours, to ensure students — already painfully behind in the Chicago Public School System — can return to school.

But don’t hold your breath. As union head Karen Lewis said on Sunday: “Our members are not happy. They want to know if there is anything more they can get.”

— Lindsey M. Burke is the Will Skillman Fellow in Education at the Heritage Foundation.