Hi JessieLB:
Even MN talks about 'a Y3 dip' (www.mumsnet.com/learning/assessment/transition-dip) - so I think in answer to your question - yes all kids do have peaks and troughs.
However, I have children at a school where to be honest I feel they're working at roughly 1 year behind their contemporaries from nursery school, who we see regularly, but who attend different local schools.
So my advice is that doing a bit more at home consistently can really turn things around.
Reading: Just really work to make this part of the family routine. We do this swap kid thing where whilst one child is having their bath the other is reading to me or DH. We also have made a point of saying no movie/ DVD until you've read the book - works great for things like Harry Potter series or Narnia chronicles. We also have taken the time to read our childhood favourites to our children. So right now DH is reading the Hobbit to DDs - like me they wonder where are the girls - but fortunately the new trilogy films have taken a bit of literary license.
Literacy: We've helped here by accident with the Collins Handwriting workbooks which we bought because DD2's teacher was forever telling her her handwriting was awful. Although it is about improving 'joined up writing' whislt you're doing it there are all sorts of spelling rules, grammatical work going on which is a lovely fringe benefit. We also bought the GCP literacy workbooks (Y4/ Y5) which DD1 worked through in the run up to the 11+ which have turned out to be useful with the introduction of the Spelling & Grammar Test in Y6.
I am that kind of evil Mum who has DDs write thank you letters for presents, treats, special days with Grandma, etc... I also don't let them enter competitions by e-mail - I make them write a letter and do it by snail mail. I have also encouraged them to keep a diary. I have a friend who's DS won't do any of the above but will design cartoons - so she encourages extra make captions for his cartoons. He has books worth now - and it really has improved his vocabulary, punctuation & confidence.
Numeracy: If number skills are weak or your not sure how to support next steps - there are a lot of on-line resources out there.
Woodlands Junior school Maths Zone is just a fantastic free resource where you can pretty well look up any area of maths & find a video game to give your DC more practice: resources.woodlands-junior.kent.sch.uk/maths/
Cool Maths for Kids is also brilliant: www.coolmath4kids.com/ - there are lessons/ explanations - but there are a ton of games to play as well - also free.
Multliplication.com www.multiplication.com/ - is a great free resource for learning your times tables - my DDs like the games section espsecially.
If you want to help with learning times tables/ inverse multiplication facts try the free version of Timez Attack & their new division facts games: www.bigbrainz.com/ - you're cast as an ogre who runs through a dungeon or castle solving multiplication problems. The game tests your ability at the start & builds from there. Multipication problems are shown as both multiple additions & old fashioned vertical problems. You are regularly quizzed by a medium-sized ogre every 3-4 problems and every now and then a giant ogre comes out and reviews everything you've done so far. Things you get consistently wrong will appear again and again until you get them consistently right - so no avoiding 7 x 8, or whatever times table fact you find tricky.
MN had a thing about Maths Champs a while back - it has games broken across age boundaries which can be upsetting if you're 9 and struggling so playing 5 - 7 games - but just assure them that it doesn't matter - again it is free and it helps build up speed/ confidence: www.mathschamps.co.uk/#home
If your school uses something like My Maths - there are all sorts of games on it which you can play to your hearts content at home.
Now we opted to join Mathsfactor, but there are many on-line tutorials out there. Khan Academy is free and from Year 4 (US Grade 3) you can follow US curriculum in maths for free on their website.
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Basically - by doing about 1 hour of extra maths a week + a bit of grammar workbooks (often whilst waiting for sister at ballet lessons) + regular nightly reading DD1 has improved beyond all recognition. It probably amounts to about an extra 2-3 hours of work over the week, but DD1 no longer considers reading work and has a tendency to play maths games for fun.
Our view is that our school was fine socially but a bit of a mess academically - with endless new initiatives being rolled out not very well and then abandoned. Homework is non-existent at our school - so we do have the capacity as a family to do a bit more as well. Certainly we've found that little and often does bit by bit lead to huge improvements in the long term.
HTH