american school system 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.

Candidates for Pre-Kindergarten – Grade 1 will be placed strictly by age. Placement in Grades 2-12 is determined by evaluating the student’s age, development, and previous school experience. The final decision will be made by the School Principal after review and evaluation of the child’s application.
It is important that students have completed a full grade level (American system equivalent) before advancing to the next grade level. If a student has not completed a grade level when enrolling at ASW, he/she will be placed in the same grade level to ensure completion of that grade level.

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References:

http://www.aswarsaw.org/admissions/grade-placement
http://www.goodschoolsguide.co.uk/international/transitions/comparative-ages-grades-and-exams-US-vs-UK
http://www.tvwise.co.uk/2019/06/sky-witness-sets-uk-premiere-date-for-elementary-season-7/

high school ages

Enrolments are now open. Contact your local public school to apply to enrol. Please return your application to your local school by Friday 24 July 2020.
All children of compulsory school age must be enrolled in school and attend every day – this is the law. All children must attend school up until they are 16 years old. Sixteen and 17 year olds must be enrolled in school or a training organisation, be employed or be in a combination of school/training/employment.

This means that a child who has turned five on or by 1 January in any year must start Prep in that year, or be provided with approved home education.
Every child, from the year after they turn 5 must be enrolled at and attending school, or an approved home education program. Children can also attend Kindergarten from 4 years of age.

References:

http://www.education.tas.gov.au/parents-carers/parent-fact-sheets/age-child-startleave-school/
http://www.gov.uk/know-when-you-can-leave-school