A High School Diploma is the official certificate awarded to students in the United States when they complete their secondary education, usually from Grade 9 to Grade 12 (ages 14–18, UK Y10-13).
It shows that a student has met the minimum academic requirements set by their state’s education department or a private school/umbrella school.
There are no universal number of requirements (the minimum is set by the state or school) but students generally need to earn passing grades in:
English (4 years)
Mathematics (Algebra I, Algebra II, Geometry, etc.)
Science (e.g. Biology, Chemistry)
Social Studies/History (e.g. U.S. History, Civics, Human Geography)
Physical Education
Electives (e.g. Art, Music, Foreign Language, Technology)
Students accumulate number of credits (e.g. 20–24 depending on those state or umbrella school requirements) across different subjects, with one full-year course in one subject usually counting as 1 credit. (This works out as ~5/6 credits a year.)
In many U.S. states, homeschooling parents or umbrella schools can issue a diploma if the student has met comparable requirements. A parent-issued diploma is legally valid in the States and widely accepted by colleges, employers and the military, particularly when supported by transcripts, SAT/ACT/AP exam scores and/or a portfolio. I recommend that home educators in the UK use an umbrella school for at least the senior year so that the diploma can be externally verified and easily recognised by UK workplace and educational groups.
[Please note that a HSD is not the same as a GED. The GED (General Educational Development) is a separate test for those who didn’t finish high school traditionally but who want to demonstrate equivalent academic ability.]