If you want to improve you're going to have to be realistic. What I mean is a month isn't enough time to improve and get to where you want to be unless you're speedy gonzales and can whip up 100 detailed high quality pieces a day.
A month isn't being realistic.
Just do as many as you can in a day. After school, go home and work on atleast 3 to 5 pieces a day. Put in details, work at 400 dpi or 600 dpi. Etc. Ask around on what to do here and there on specific question that you might have.
Improvement takes much longer then a month. It can take years if you're only going at a rate of a few pieces a day. And to be honest if you rush it and do more, then you'll just be setting yourself up to be only capable of handling speed colorist jobs. Very sloppy work with rushed results. Nothing special about that except for getting the job done as quickly as possible.
Take your time and learn right from what works and what doesn't.