As the head does not appear to be addressing it, I think I would contact the Chair of the Governors and ask why the teacher did not appear competent to teach in a differentiated manner - extension work should be available for brighter children - it is part of the Ofsted Inspection to observe how well staff extend kids. Schools (I believe) are also supposed to show progression - by this I mean 1.5-2 sub levels a year (unless there are very good reasons why a child has not achieved this) from the Y2 SATS to the Y6 SATs and, in the spirit of parent/school cooperation I cannot see why a school would say it is just for the teachers to know!! (I can understand maybe not giving exact sub levels but a comment like "a comfortable Level 4" would indicate a level 4B, for example.)
In our school we only get one written report a year and have two parents evenings in the year where we can speak to the teacher for about 10 minutes. Even with this, in the parents evenings, we always discuss levels so as to get an idea of where our son is at and where he needs to work harder.
Agree with kenyancowgirl - it may be worth your while to cc the parent governors also (in our school, issues raised by parents with the Chair and the Head were often _not_ reported back to the governing body, so other members did not know what issues where floating around...
Throughout our time at primary, all the teachers have been very specific with the levels, telling us their sub-levels for reading, writing and maths; indicating what areas need work, what work needs to be done to get to the next (sub)level etc. I've never heard of a school refusing to discuss these.