Wiktionary
tenth grade
n. (context US education English) The period in school that comes after ninth grade and before eleventh grade.
Wikipedia
Tenth grade
Tenth grade (called Grade 10 in some regions) is the tenth year of school post- kindergarten in many parts of the world. The students are 15–16 years of age, depending on when their birthday occurs. The variants of 10th grade in various nations is described below.