Poland's improved significantly in international education achievement tests in the last decade. This paper examines why.

Most important part of the reform is the delay of vocational education/stratification/allocation of pupils with the introduction of a comprehensive lower secundary education program and more instruction time.

OECD; 2010; Impact of the 1999 Education Reform in Poland; Paris; OECD - Education Working Paper no. 49.

The conclusions are somewhat directive but interesting to take notice of, p. 26:

Our results suggest that, on average, vocational schooling reduces test scores by a full standard deviation. While other aspects of the reform programme no doubt helped improve Poland’s PISA scores, delayed entry into vocational education played a major role. We argue that the way to achieve better PISA scores is through more hours of instruction, greater exposure to testing, and increased student and teacher motivation.

We substantiated our findings by taking advantage of the application of PISA to 16 and 17-year-olds. We find that once vocational school options are available again, when students are 16, test scores decline for those students who enter the vocational track. While this goes a long way towards proving our initial findings, it also serves as a caution to policy makers about the effectiveness of vocational schooling, particularly when that schooling is not designed to improve math and reading skills. Those are skills that all students can learn, if given the opportunity; they are also the real vocational skills in the world of work today.