Mayor Bloomberg and Chancellor Joel Klein are moving toward bringing a truly revolutionary concept to the public schools: accountability for improving student performance.

They have decided to begin grading schools on how well principals and teachers lift achievement as a way to hold them responsible for the learning they manage to impart. Imagine that - putting the adults to the test, not just the children.

This is a fabulous idea that could do more to overhaul New York's schools than all of Bloomberg's previous reforms combined. If executed properly - no small task, considering the data that need crunching - administrators would gain the ability to identify which principals and which teachers speed educational progress and which don't. Parents would know, too.

Never has this happened in New York, as it was generally accepted that half the students would fail to graduate through no fault of anyone in particular. The system at large was to blame, along with social ills, not individual teachers or principals.

What Klein has hit on is the concept of applying objective standards to measure educators by what they produce, just as most workers and businesses are judged by the quality of their output. School leaders in a number of cities are taking similar approaches, but none on such an extensive basis.

The system will track all 1.1 million students from grade to grade, creating a record of each child's progress. If a kid stops learning, that will show up in the data. So, too, if a struggling child starts to catch up. Even better, the system will allow direct comparisons among teachers in the same grade, and among schools with similar populations. Schools will be graded, A through F, based on factors including standardized test scores, attendance rates and safety. Anyone can log on and see how a school is doing.

Who could be against that? The principals union, for one. Boss Jill Levy calls the plan a sword of Damocles over principals' heads, which merely reflects the union's revulsion at being judged by performance. Perhaps, too, Levy sees the pressure to reform contracts that are written to protect the lowest common denominator, rather than to make sure children actually learn.

Klein's project is a long way from completion. It will require new computers and new software and lots of training. And it will be costly - some $20 million for the high-tech data system alone. But it is worth every effort. You need smart data to make smart decisions, and this is a solid investment in the future.

(c) New York Daily News, L.P.: reproduced with permission.