Regarding what means Don't leave it for tomorrow, it means doing things at the moment and not later.
On the one hand, I think that students tend to procrastinate everything or almost everything that they have to do because they have other things to spend time on them. Teens usually say that they have to meet friends, go out, go to parties, and now in summer, they rather go to the beach than stay at home doing something. Most of them say that among other smells, the smell of sunbathe reminds them of their childhood.
On the other hand, it is true that if you start now you will be ready for the next school year or whatever. I rather do some homework during the summer than have to do all of them during the school year, when I have to do a lots things more.
All in all, if you can do things at the moment, do them, because later you might not be able to do them because of different things.