Not having a bag in school is seen as quite cool: the bagless are often seen as mavericks who do as they please and answer to no-one.
But you’re not in school now, are you? You probably do need a bag to carry your stuff around in.
The Fjallraven Kanken is a perfect bag for the bag-reluctant. Big enough to fit in a butty and a book, but not so cumbersome that you can’t just carry it by the handle, all loosey-goosey.
And they age wonderfully. A bag for life.