Linda's answers are good. However often more casually it'll be: "She went to high school with me" "We went to high school together"

Well, if she has once been your high school classmate, she'll always be your high school classmate, although she may not be your classmate anymore. So, if you have graduated from high school and she's not your classmate anymore, then "she is my high school classmate" works, as much as "she was my classmate in high school (during high school years)". Anyway, I usually go with "this is (name), my high school classmate", which avoids the fuss.