Smoked weed for the first time when I was 13. Continued to smoke and do other drugs regularly in high school. My SAT score taken when I was 17, going on 18, was just shy of 2000. You can experiment and not have it ruin your life.
If you're concerned, say so, but leave it at that. You can't stop her. Fighting it will, at best, have no effect at all, and at worst you could push her into doing drugs just to spite you.
Edit: This thread isn't about how harmless weed is. People who abuse weed to the point of neglecting other responsibilities can end up in just as bad of a position as people who do real drugs. Sure, you won't die from weed, but having a shitty life because you chose to smoke weed instead of having goals and interests outside of drugs is a real possibility. There's nothing wrong with the OP being concerned.