The ages for education/schools in the UK:

     5-11    -> Primary School
    11-16    -> Secondary School
    16-18    -> Further Education (often called college, or Sixth Form)
    18-21/22 -> Higher Education (University/undergraduate)

Some secondary schools are large enough/have enough expertise to have a Sixth Form as part of it. Some children go to "preparatory school" until age 13 when they enter a secondary school.

Yes, in the UK high school starts in year 7 (age 11-12)