If it's true that GiveDirectly doubles the income of a family of 4 for one year for ~$1000, then a high school student can achieve that by working 100 hours in a job that pays $10/hr. It's unclear that most high school students could do better than this through volunteering 100 hours in a year. I don't think that GiveDirectly is actually that cost-effective (despite the fact that it's an informed estimate), but I would not be surprised if it's sufficiently cost-effective anyway.

Still, I would guess that high-skilled high school students can do better than earning and donating.