The KIPP students who graduated from college were not the academic stars but the workhorses, the ones who plugged away at problems and resolved to do better.
" To plug (plug) away" means to continue to do something, especially to continue to do something that may not seem important that you may have to do over and over again, but that has to be done.
But the thing that was really important in Germany was the idea that as like a bright but fairly normal bourgeois German young man, you could become a chemist and plug away at doing chemistry work.