Sometimes the commitment it takes to get in shape is much more than you anticipated when you first start. There is no way to determine what the future is going to bring and it seems like, the moment you make the decision to get into shape, the future brings a lot more things for you to do each day. This is why so many people start off like gangbusters when they first start going to the gym and end up paying for a membership they never use. It's not because they're losers for failures, it's because life sometimes get's in the way of the things we want to do each day. If this happens to you, don't give up, just regroup and restart on your way to your goal again.
If you find you're skipping days repeatedly, you may want to invest in some sort of equipment to keep at your home. Find something you can jump on, get a quick workout and get off without too much set up or tear down time. Most people find elliptical exercise equipment to be perfect for this type of situation. An elliptical will give you a great low-impact cardiovascular workout in a very short time. With the arm attachments, you're getting your entire body into the process. Even if you do visit the gym regularly, it may be a smart investment to use each morning to give your metabolism a little kick start at the beginning of each day.
There are a lot of students who are able to squeeze in some extra reading time while they're working out rather than sitting at the local Starbucks. Grab your textbook and jump on a recumbent bike or even the elliptical and begin reading. You can also flip through your flashcards as you're working out. If you're work is requiring more of your time, do the same thing in your situation. Try to isolate some of your work tasks that can be accomplished while you're burning a few calories on your stationary bike.