an educational course that happens during the summer holidays

They organize an annual summer school for young musicians. 他们为青年乐手组织一年一度的暑期课程。

one or more classes taken during the summer that replace classes that were missed or failed, or make it possible for students to move more quickly towards a degree or graduation

The students had improved enough after summer school to be promoted to 4th grade. 这些学生在暑期补习班提高了很多，有资格升入四年级了。