Talk to the older pupils if there are any around, often its the year 7's who are there to tell you how great it is and the novelty hasn't worn off for them yet, also speak to parents at your child's current school who have older children and ask both the child's and the parents view of the school if any attend, that can be a real eye opener.
We have 2 local schools, one has been a very popular choice and always oversubscribed, but 9 out of 10 kids I have spoken to and their parents have quite a few issues. Another which has had a bad reputation but has really turned around recently but I have not found a single person who currently attends who says a bad word about the place.
Ask about setting/streaming. Is it done? From what age? Once decided is it permanently fixed? Having been keen to learn but at a comp with slightly below average results, i would be very wary of a school with little or no setting for the first few years.
You can probably skip the headteacher's talk. Chat to the Y9/Y10/Y11 pupils. Are they the sort of young people you would like your Y6 child to become? How positive do they seem about the school?
I would want to know which subjects the school offers at GCSE.
I would be looking for a school where almost all pupils sit almost all of their GCSEs at the end of Y11, not half of them at the end of Y10 (thereby getting the C the school needs for the league table but not the A*/A/B they could get at the end of Y11).
I would be looking for a school that doesn't put pupils in general ability streams, but does put them in sets, particularly for Maths and MFL.
I would ideally like a system for the older children mentoring or helping the younger ones.