What would be a good high school graduation gift for a girl who wants to be a grafic artist?

Graphic art supplies, of course

Graphic Art Supplies - Rex Art Supplies

An airbrush or a set of mechanical pens.

Anything artistic, or a gift card so she can get what she needs.

I would say a gift card, many artists are very picky as to brands and types. And of course money in and of itself, is never a wrong gift, since artist have other needs also.