Why did Jacob drop out of school?

Well, most of Eclipse took place over the summer.

As for Breaking Dawn, I have no idea, I didn't really pay attention, but I guess there was no mention of school, and he was always doing other things.

It diddnt dirently say that he dropped out , but we can just assume that if he did, then if you were in his case , you would too.
When you transform into a supernatural creature with the circumstances of a vampire returning to hurt the ones you pag-ibig then maybe school would not be your tuktok priority.
And also what cassie said, summer holidays and all that tiome inbetween.