Yes, I found when I was at university that it was very difficult to take on a personal project and still be motivated to do my school work.

I would start hacking on something and not notice the time going by, so I would end up having to do assignments the day before. This was easy to begin with since the course starts easy but after a while becomes impossible.

The only way I can see it being possible would either be to have super high intelligence and productivity or military level discipline.

That's why I am amazed to hear of people who start whole companies whilst still getting top grades.