Hi Judith, looks like Mike beat me to the Mr Chicken correction, but I won't cluck at you for it! (lol, that was lame).
I was a full-time student as well just up until a few weeks ago, so I know the grind. The most important thing is discipline. I was determined to go to the gym 4 times per week and I blocked out the days and times I planned to go so that I could plan the rest of my study blocks / appointments around those times. I also found that for me, a cup of plain black coffee could stave off hunger and last me a long time so that if I kept one with me while studying, I wouldn't feel the need to munch mindlessly on something that would just end up accumulating on my body.
Exercise and diet are both components of losing fat. In my experience diet accounts for 70-80% of that loss, and exercise the difference. If you've got a stubborn final 15 then simply more exercise might be the key to putting it away for good.
With regard to relaxation, I shoot for at minimum 7 hours of sleep a night. It can be tough as a student, but again, it comes down to discipline. I used to always want my Internet time at the end of the day so I would finish studying at 11PM, then be online until 2AM regularly, until I realized that the Internet isn't going anywhere and there's too much of it for me to ever cover in my lifetime, so I might as well sleep more.
If I find myself sitting for extended periods of time, I'll get up and walk around a bit, then do some lunge stretches for my hips and back, which can really loosen you up and wake up your brain too. For at least an hour a day, I try to do something not school/work related, like reading for pleasure or working on a hobby. It really helps to relieve stress put on you by school and work and just gives you space to breathe a little and live in the moment, which is something I believe is important for optimal health.
1) eat real food - more vegetables, moderate meat, moderate fruits, less grains, less sugar, less vegetable oils.
2) exercise - moderate intensity cardio, sprinting, heavy lifting, dedicated stretching and mobility.
3) live - relax, de-stress, meditate.
Disclaimer: I'm not professionally qualified to make any formal recommendations. I've just done my homework and I'm my own guinea pig. All of my data, unless otherwise cited, comes from a sample size of n=1 (me).