When do you capitalize the word senior as in Senior high school?

You capitalize "Senior" when it's part of a proper noun, specifically referring to a formal name for a grade level. Therefore, "Senior High School" is capitalized because it's a proper noun designating a specific school or school level. However, if you were to say "She's a senior in high school," then "senior" is not capitalized because it's a common noun describing her grade level.