Which, comma or no comma before?

Hi,
http://www.guardian.co.uk/education/2009/aug/16/harder-a-levels-gove-conservative

• An end to the government's current measure of GCSE performance which judges a school on the proportion of its pupils who gain five A* to C grades.

I think that "which judges a school on the proportion of its pupils who gain five A* to C grades" is an extra piece of information unnecessary for determining which measure of GCSE performance is referred to here, so a comma is needed before "which".

Is a comma needed?

Thanks in advance