Hi Spice,
I think all schools are different but I think the KS1 sats results do make a difference to your childs schooling, my daughter got level 3 in her sats and you do seem to find they put all the higher attaining children together and seem to give them tougher work etc.
I think that what level your child attains in KS1 sats are a good indication of their ability to do well in the 11-plus. My daughter has just sat the 11-plus, still have a couple of months till we find out the results

.we have no idea how she got on but thought it was worth a try and she was keen to do it.
But generally I think people say that when your child is in year 5, ask their teacher what level your child in expected to get in the Ks2 sats , if its level 5 then it might be worth having a go at the 11-plus. Good luck!!
