I'm not a natural planner, but I learned to appreciate plans the hard way. Planning requires a conscious effort, but if you do it regularly, it shouldn't take more than a few minutes a day, and it pays off.
It's easy to brush it off as one more thing you don't have time for, but those are the times you need it most.
Planning encourages you to think concretely about what your goals are -- always a good step. Break the goals down into tasks. Once you get them out of your head and on to a piece of paper, you can focus on them more clearly: count them; prioritize them; validate them; make a list of what needs to be done to accomplish each one, and by when. Break big tasks down into little ones. Estimate how long it will take to finish each one. If you're not sure, multiply your time estimates by 2. Add them all up. If they adds up to more time than you have, take a closer look at your estimates. Are there "must dos" and "nice to dos"? See if there are places where you can streamline. Knowledge is power.
If your list is like the one I made when I was trying to finish my doctoral dissertation, you'll start to notice patterns. Some things won't take very long at all and won't need intense concentration. Others will require longer periods of more concentrated thought. You can start to visualize your day (or your week) as a series of smaller and larger blocks of time. By planning your tasks, you'll be better able to take advantage of small blocks of time, because now you know exactly what needs to be done.
When I first did this, I was overwhelmed at the thought of everything that had to be done to finish my doctoral dissertation. By creating a plan, I was able to convince myself that by doing just two hours of work a day (on top of my day job), I would have more than enough time to finish it by the deadline. Suddenly I was using my time better. I was checking references and proofreading chapter drafts on subway cars, and crossing tasks off my list.
It was liberating, and I suddenly started feeling more confident. If I completed my two hours a day, I could relax without guilt knowing that I was still on track to meet my goal. I started noticing that my time estimates were too generous; I was finishing things in half to three-quarters of the time budget. But that was okay. For me, the primary purpose of those estimates was to convince myself that I had enough time to do the job. And I did. I finished, in plenty of time.
So, here my recommendation. If you want to do well in school, and you only have a few minutes a day, make a plan. It doesn't have to be fancy: a planning book is nice, but a piece of paper and a pencil will do. Think ahead, make your goals concrete, list your tasks and the resources you need, and make the best use of the minutes you have.