Improving grades and being more productive are two separate things. You can have good grades and not be productive or vice versa. As such, here are two helpful links to be taken with a grain of salt:
How to improve your grades from WikiHow. This is by no means a definitive answer, but it's a start and I like the basic message to try your hardest and don't be afraid to seek school help if you need it. Oh and has anyone noticed the similarities between WikiHow and Ask.com? I know that WikiHow is collaborative, but the two services otherwise seem the same.
How to be more productive from Wired. This is also not a complete answer, though the main message is that organizing oneself will lead to more productivity. Though this is a commonly held belief, I remember thumbing through A Perfect Mess and thinking Eric and David made some good points which would counter the Wired article I just provided.
A review of A Perfect Mess from Amazon:
Flying utterly in the face of conventional wisdom, the authors turn the world of organization on its head to examine how messy systems can be more effective than highly organized ones. Neatness for its own sake, they say, not only has hidden costs in terms of man-hours that could be spent doing other work but it turns out that the highly touted advantages may not even exist. More loosely defined, moderately disorganized people and businesses seem to be more efficient, more robust, and more creative than the obsessively neat.