Find something inconspicuous to work on during your classes. For me, this was usually math teasers of the sort you might find at a math competition. Copy some d

Find something inconspicuous to work on during your classes. For me, this was usually math teasers of the sort you might find at a math competition. Copy some down, work on what you can. There are a lot of forums online that provide questions of all different levels of difficulty and all levels of accessibility. (I have unfortunately lost the link, but there is a particularly good site that tends toward puzzles that require little more than simple probability and logic.)