Need help for this analytic problem:

Students test scores on general education were sampled across a states school districts and schools. Class size (a dummy coded variable indicating smaller to very larger class sizes) was also collected. Determine the impact of class size on general education scores. Be sure to perform a analysis and provide a full summary.

I tried to use lmer , but figured that data is not a balanced design, Can someone advise me via R studio?

I checked the skewness of educationscore and solved the skewness by logeducationscore+0.2)

Dataset is an excel file, I can't upload, so I have to post the file link here.

https://drive.google.com/open?id=1_ovhvrrFVNV2gP8c5vrg1FZ8C9x9NqCT