To be admitted, you of course have some minimum requirements: a certain number of math credits, a certain number of language credits, etc and are required to submit a transcript. However, a diploma is not required, especially in the higher tier schools.
International students usually just need to have their application processed through an international education certifying agency.
They also require other signs and signals besides a SAT that foreign students who’ve come to America as citizens (and study abroad students) were never prepared for and don’t have…such as certified extra credit, letters of recommendation…etc.
What is “certified extra credit”? Admissions requirements for international students are almost always the same as for domestic students, with the exception of a TOEFL score often required for those from non-English speaking countries, and external certification (and usually GPA conversion) of secondary education work.
Yes, private schools can be more flexible with their admission standards but 99% still want to see that diploma.
Absolute not true. Both public and private schools will admit without a high school diploma.