On a school day, I don't really have much time to spend doing non-academic stuff - ie. watch TV, go shopping (not that you would expect this) and the same applies to exercising. On such days, when I have limited time, this means that I need to squeeze in a decent-intensity workout into around 45 minutes. I have followed this plan of going down to the gym for about 45 minutes every weekday for the last month and it has worked perfectly. The workout is short enough that I am not spending too much time in the gym, yet long enough to get my heart rate up and keep me fit... and that's why I love it. It took weeks of experimenting and trial to find a routine that worked for me and one that I wouldn't get bored of.
Recently, I have started listening to music when I workout (I never used to!) and realized that it really helps me keep focused and motivated. Also, science has proven that the beat of the music keeps your motion and rhythm and takes your mind of your breath (panting..). I have made a playlist with 10 songs that work for me and it comes to just about 45 minutes. This routine has set in so well that going to the gym is actually a great relieve from school work and I'm genuinely excited to get moving and sweaty.
20 minutes on the treadmill (around 3.5km)
15 minutes on the stationary bike (2 minutes high resistance; 2 minutes low resistance - then keep repeating)
5 minutes - 1)weights and arm strength 2)alternate with rowing every other day
5 minutes - ab/core workout
It gives a whole body workout and leaves you feeling re-energized and packs in a boosting punch.
Hope it works for you! Please leave me a comment if you have any other ideas and as always, there's absolutely no need to follow my formula exactly and vary it as you like so that you feel the same energy and motivation as I do!