Is a 17 year old a junior or senior in high school ?

Ummmmmm ...

It depends on the 17 year old.  And when their birthday is.

usually both, people often turn 17 during their Junior year in high school unless if they have a Summer birthday.

It depends on how many years in school they attended. The age will give you a clue but it isn't conclusive. Personally, I was born just 11 days after the cutoff date for starting school in my state. As a result I was usually the oldest of my classmates, so I was a junior at 17 and 18 as a senior. 

Yes. I finished my junior year as a 17-year-old. I started my senior year as a 17-year-old. You do realize that not all high school kids have their birthdays over summer break, don't you? Never mind that August birthdays give parents on option of what year to start their kid in kindergarten and never mind that some kids skip grades and fail grades.