When I graduated high school, I remember my grandma gave a necklace. I wished my parents was there but it was fun and memorable enough as I have my grandma and siblings around that celebrated my special days.
It doesn't matter what kind of gift for high school graduation you can get for your kids, siblings or friends, it will always be special. The most important is your love and presence and the thoughts of getting him/her something on her/his special day.
You can also check out "thesource" website for great gift ideas for high school graduations.