Complex sentence: The tenth grade is the tenth school year after Grade 9 grade 10 is a part of high school, and in most parts of the USA it is the second year of high school, with 11th then 12th following.
Split into simpler sentences: The tenth grade is the tenth school year after Grade 9. Grade 10 is a part of high school, and in most parts of the USA it is the second year of high school, with 11th then 12th following.