Maintaining/improving drawing skills without having the time to draw
The following exercises can be done with most writing or drawing utensils while focusing on other things. They will train your imagination, and will maintain your hand-eye coordination and finger and hand movements involved with drawing:

Take your utensil of choice, find an even and blank surface and doodle.
This can be done while listening to lectures or podcasts, and could even help you concentrate better.

Write differently. Instead of your regular handwriting, experiment with other types. Try writing in cursive, mirrored, with your other hand, or only using capitals, &amp;c.
This is easy to do while doing your homework or taking notes.


The following things can be done in the margins of your (note)books, or on small pieces of paper you happen to carry, like receipts. They will need temporary attention, however:

Do small exercises. For example, experiment with different kinds of hatching, try giving simple forms (spheres, cubes, pyramids, blobs) a convincing apparent three-dimensionality, combine different colours to see how they interact, or draw only residual shapes (the shapes between objects), &amp;c.

Make quick sketches of objects around you. Limit the time you spent on a single drawing. Try to alternate what you focus on: shading, proportions, volume, contrast, composition, &amp;c.

Hang up a single or a few sketches by (old) masters (e.g. Annibale Carracci, Albrecht Dürer, Francisco de Goya) where it is that you study, and observe these whenever you feel like you can use a short break from your homework or studying.
Pay attention to the lines: the accuracy and swiftness with which they were put on paper, the changes in their thickness and what they signify. Analyze the hatching, and how those lines relate to each other and to the contours of the object drawn. This will hone your observational skills, enhance your knowledge of drawing, and likely increase your urge to draw (which is either a good or a bad thing in this situation :).