In my experience in this part of the U.S. they are interchangeable. All the school records say Grade 9, Grade 10, Grade 11 and Grade 12, but people commonly use Freshman, Sophomore, Junior and senior in conversation. And not all high schools here have grade 9/Freshman. They are only 3 years. Grade 9 is can be the third year of Junior High School. My school district has grades 9-12 as high school, my wife attended the school district that borders my district, and high school is grades 10-12 there.