Global Education Indicators - dataset I've been looking for a dataset with a single education indicator by country so I can rank/compare them but I haven't found anything.

Already seen Unesco and Worldbank data but info is too much for what I really need at this point. Also had a look at the  Human Development Index (HDI)  but this is a composite statistic of life expectancy, education, and income per capita indicators so my analysi will be obscured by other elements.

I just need (for now) to be able to say for example Japan > Spain or Canada ranks 10th worldwide.

Does anyone have any idea as where I can download (if exists) this data set? API can work too. Ideally info comes with country iso code.

Thanks in advance.
You can use the PISA study from OECD. It evaluates the skills and knowledge in reading, mathematics and sciene of 15-year-old students. PISA does not combine these results, but commentators do so sometimes.  

The most recent results are from 2012, you can find an overview of the rankings here. These rankings are composited from various tabels you can download using the links at page 297 from this document.
Results of 2009 are available in a xls-file. Scroll down this artcile to download.