In my country, high school starts in grade 10, so my 9th grade GPA isn't great (8.7/10). Is it true that a student's GPA in ninth grade matters when it comes to college admissions? Is it compulsory to submit my 9th grade GPA?
The previous answer is not correct.
First, 99% of college applicants do not have a "year 13" of high school because 99% of US HS's are K-12 US. Internationally most countries end in year 12 but do sometimes stop in 10th or 11th grade depending on the compulsory education standards in your country. Some Latin American and African Nations end HS after 10th grade. The only exception is if you attend a private boarding school with a post-graduate year which is an extra year after you graduate from HS. If you only have 10 years of compulsory education, it is very difficult to qualify into a top US college.
Second, most US colleges evaluate your transcript from year 9-12. There a handful of exceptions like Stanford University that re-calc your Stanford GPA based on your reported 10th and 11th grades (and at the mid-year 12th grade when it comes available for RD admissions). All Ivy league and other Elites fully expect you to have the following coursework on your transcript, 4 years of English, 4 Years of Math, 4 years of Science, 3-4 years of language and 3 years of History. In order to fulfill those prerequisites and meet the academic threshold for these schools, you will have had to take have 9th, 10th, 11th, and part 12th on your Common App. This applies to International students as well.
Third, American colleges do not care if your high school starts in 10th or 9th or 8th grade, they require a full and complete transcript. So you will have to send both your transcript from your current school and previous school. If your HS is 4 years and starts in 10th and goes for 3 years then you supply all your current transcript and your 9th grade report from you previous school.
There many be exceptions or additions so before you actually apply to a particular college look up their admission requirements for Int'l student on their website. If it is not 100% clear, then you should contact their admissions office.
GPA will be calculated from High school so mostly from year 10 to 13, depending on the university you are entering.
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