Well, let's say the difference in charity between the teacher and the bonds trader in my example is $5000. Each year, if the teacher teaches 5 classes with 20 students each, then he probably has 100 students. If 100 students have a significantly better education that year then they otherwise would, then I would think that would be worth more utility in the long run then $50 per student, wouldn't you?
We don't want to compare education with dollars here, we want to compare education with the best thing you can easily buy with a charitable donation, and that's probably mosquito nets which save one child's life per $2500.
A very interesting article on "earning to give", featuring LessWrong members Jeff Kaufman, Julia Wise, Holden Karnofsky, William MacAskill and Toby Ord. Some excerpts: