High School lasts 3 years in my country, do I have to submit my grades from my last year in middle school?
You need to ask the international admission offoce at each place you apply to about this. Do not be surprised if each answer is different. Just send what A wants to A and what B wants to B.
My friends this year submitted 8th grades to every school they applied, but again check again with admission office.
Yes. You need to submit your last 4 years of secondary school. Your school profile or transcript must also make clear the level (you may have to approximate but all countries have different versions of classes, like ‘math for students who will do a vocational diploma’, ‘math for students who won’t continue’, ‘math for students who plan to go to college’… Indicate that level.) And if a class is a continuation it needs to be made explicit ie., If you took German in grade 7&8 your German class in 9th grade must make clear it’s the 3rd level. Or if you have Integrated Math 1-2-3 and 3 is in 9th grade you have to make that clear.