求救！

An essay not submitted in class on the due date will lose a letter grade for each class period it is late.

句子里的it is late 在句子里算什么？

可不可以理解为 for each class period it is late是插入语？

An essay not submitted in class on the due date, for each class period it is late, will lose a letter grade.

如果是的话，插入语是个句子的时候是可以放在句末省去逗号的吗？