how old are you when you start high school uk

Your child must start full-time education once they reach compulsory school age. This is on 31 December, 31 March or 31 August following their fifth birthday – whichever comes first. If your child’s fifth birthday is on one of those dates then they reach compulsory school age on that date.
Children must stay in full-time education until they reach school leaving age.

At the age of five or six, the children attend elementary school (also known as grade school or grammar school), which last six years. The fist year at elementary school is called kindergarten.
In the USA, children start school when they are five or six years old. Depending on the state, schooling is compulsory until the age of 16 or 18. Children younger than five can go to a nursery school or preschool.

Our sister school, Cheadle Hulme High School, is an outstanding and hugely oversubscribed academy with a reputation for the relentless pursuit of excellence and a determination to ensure that every child succeeds. Didsbury High School will embed the same vision and values through our partnership and aim to deliver the same outstanding education.
We are very excited to welcome you to Didsbury High School, a brand new secondary school which opened in September 2019, and a member of the Laurus Trust family.

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References:

http://www.ego4u.com/en/read-on/countries/usa/school
http://www.didsburyhighschool.org.uk/
http://www.goodschoolsguide.co.uk/choosing-a-school/northern-ireland-education-system
http://prosperoteaching.com/from-overseas/uk-key-stages-by-age-year/