Working out two days in a row, then nothing all week is definitely not ideal. There is no absolute set amount for everyone, b/c everyone is different.
Also, just b/c you don't have time to officially "workout" doesn't mean you can't exercise.
*Try sitting on a stability ball at your desk while doing homework (it's good for your posture, back muscles, and abs)
*Be one of those people who actually do park further away in the parking lot to walk more
*and take the stairs instead of the elevator (walking up stairs with books is great exercise)
*everyone has 5 spare minutes at some point during the day, when you do, do a couple of sets of squats or lunges, the next time do pushups, then dips - you get the idea
*get some of those resistance bands - they are totally portable and you can do a set or two of many exercises in no time with those