Children who're 5 years old on the 1st September go into year 1. Then it's sequential from there.
So if the girl will be 11 before the start of the new school year (on 1st September) she'll go into year 7. If she's not had her 11th birthday by then, she'll go into Year 6.
In some areas (where the 'traditional' pattern of education, as created by the 1944 Education Act) still applies, schools are divided into:
Infant (Years 1 & 2)
Junior (Years 3 to 6)
Secondary (Years 7 to 11, or 13 if there's post-compulsory provision)
If the girl moves into one of those areas, she'd go to a secondary school. (Such schools often, but not always, incorporate the word 'high' into their title).
In other parts of England there are middle schools. In most of Sheffield, for example, this system applies:
First (Years 1 to 3)
Middle (Years 4 to 7)
Secondary (Years 8 to 11. Post-compulsory provision is in separate tertiary colleges).
In Suffolk though, this currently applies (even though a return to the 'traditional' system is planned):
First (Years 1 to 4)
Middle (Years 5 to 8)
Secondary (Years 9 to 11/13)
So there are some parts of the country where an 11 year old girl would have to wait one or two years before joining a secondary school.