I am with you tryingtobeMaryPoppins2! I am not overly stressed but am concerned and taking it all very seriously! I have done 6 primary school visits, 7 if you count one I went to twice. One private all the others state (well one is c of e). Most are in or have been in special measures. Several have a change in head or will be or the head has just come back from long-term sick.
So I compiled a spreadsheet to try and score. It has categories like academia, environment, community, resources/facilities, building, management and oversight, things the school does over and above and extra-curricular, overall impression and each section is score in importance then each of the 10 or so questions under each subject have been scored in order of importance for what my little one needs!
Goodness knows what I'll do when it comes to senior schools!
I have a fair idea what we will put as first choice but places are so over subscribed i have to choose the three very carefully.
I agree with 5moreminutes that it is mainly about developing social skills but I believe they do need encouraging in their interests and to learn within school. If they are not challenged (in a positive way) then they will become lazy to learn and won't do so well in senior school. Equally if the class is too disruptive and they can't learn or aren't noticed then it wouldn't be so good.
So I guess my spreadsheet could be condensed to will they learn something new on a daily basis, will they make friends, will they be safe, will they enjoy going there? In which case I think I can't go too wrong and needn't get too panicked. 
All the best.