I have two datasets of schoolchildren performance by height in cm, one for each school. I was wondering how to best visualize how the differences in the performance between the schools vary by grade. I was thinking of creating a regression for both school's performances, and graphing the differences between the two lines, but I'm not sure how rigorous or informative that would be.
You question is really abstract... I'd propose to try different visualizations and try to decide what exactly you want to show.
I leave here a small example that could be converted, as I think, to your needs (color = school, price = performance, carat = grade):
ggplot(data = diamonds, aes(carat, price, color = color))+ geom_point()+ geom_smooth(method = "lm", se = FALSE)