One thing to remember is that schools sometimes look at the trend - if you start out weak as a freshman and then improve over time, the school may take that into account.
My opinion is that the key to good grades is to start out fast each grading period. Really work extra hard to do well on the first few assignments - that will set the tone for the rest of the grading period.
Obviously if you are in the middle of the period now and are asking how do I get out of the hole I have dug, this advice won't do you much good. All you can do is really focus on the work at hand, go in early before school or at lunch and get extra help if you need it.
And don't ever skip an assignment - always take a shot at it - even getting 50% on an assignment beats a zero - and the teacher will know you tried.
Hope this helps