Study up on the school before going.
Look at trajectory of SATs results over several years (at KS1 & KS2): info here: 1994- 2011 performance: www.education.gov.uk/schools/performance/archive/index.shtml
2012 performance: www.education.gov.uk/schools/performance/
You want to look for steady results or progressively improving results - if you see a school which is all over the shop - that doesn't bode well.
2013 results won't be publically available for individual schools until Dec - but if a school isn't openly letting you know what last year's results are - that probably isn't a good sign - however, you can always ask at a meeting.
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Look into where the HT is in their cycle. Our school will have our HT retire in the next year or so - now many are pleased about that and we hope things improve at the school with a fresh HT - but it can be the obverse - a great head retires to be replaced by a mediocre/ unpopular/ bullying HT - and the school then goes down hill.
Sometimes there is no avoiding it - but be aware that a HT retiring can be unsettling for a school.
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Does the school seem organised?
Is the website up to date?
Do they have digital whiteboards in classrooms?
Do they have ipads/ tablets/ computers?
If yes to all of the above - ask yourself - did you actually see these gadgets being used during your tour? If they are actively being used and showing that kind of thing off to you - that's a good sign. Even better if you see children happily working away on tablets/ computers/ etc...
Do they have after school activities?
Do they have school dinners? How much?
Do they have music programme?
Is there childcare on offer before/ after school or they have something set up with an independent child carer (e.g. the church associated with the school has before/ after school clubs).
Do they use twitter/ group e-mails to notify parents of important events/ cancelled school due to inclement weather/ arrival times back from field trips/ etc...
How are the office staff? Approachable? Friendly? Professional?
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When you're touring the school consider bringing your DC along - some schools have little sessions with prospective students whilst letting parents tour the school - and it is really interesting to get your child's opinion of the experience. (DD1 had very definite likes/ dislikes - definitely didn't like shouty lady).
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Ask friends/ neighbours about the school - or go to local park and strike up conversation (Where does your son go to school? Oh - we're thinking of applying there - are you happy with the school? - if you note major hesitation - that's not a good sign.
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Check out parent view on your prospective schools: parentview.ofsted.gov.uk/parent-view-results
Finally - relax - there is no making the perfect decision. 7 years is a long time and there will be teachers/ aspects of the school system that drive you crazy (endless requests for money being top of many people's lists) - so try and go for the school that ticks the most boxes
and good luck!
HTH