American universities require students applying to submit a transcript of their high school (last four years) scores. Although these are not typically produced by UK schools, you will still need one to apply.
Your transcript should be a one-page report on school letterhead detailing chronological scores or predictive scores from years 10-13 as well as GCSEs, A-levels, etc. The transcript should be signed by your headmaster or a school official and stamped with a school seal.
The basic purpose of your transcript is to demonstrate to universities that you have a proven track record of academic results.
A sample transcript is posted below.