Pitchounette - I'll have a go, but since I'm in Scotland I haven't actually experiences the English system myself. (I'm assuming you mena England, because you mention "reception" which we don't have up here).
You can find out what the children are expected to cover from the [http://curriculum.qca.org.uk/ National Curriculum website]]
SATs are only used to grade the schools and teachers, but some make so much of a big deal about them that children and parents get stressed.
G&T stands for "Gifted & Talented" - each school is supposed to identify the top percentage of their children and make special provision to ensure they are properly stretched. But in reality it varies between schools - some don't do it at all, because they think it divisive, others label the children but don't make much special provision.
Skipping a year sometimes happens, but secondary schools often will not take childern "out of year", so they might end up having to repeat a year later in primary school, which would usually be more difficult for them socially, so it doesn't happen very often.