First, there is always possible to improve your marks if you felt that the percentage you secured in your inter exam or term end exam marks are too low or average to be taken into consideration.
If one is extremely unsatisfied with the marks, you can ask for reevaluation of your mark sheet before the time run runs out. Write and Submit a written application to the head of your school, mentioning the previous marks you secured and want to improve. They will be more than happy to accept the proposal.
This is highly not recommended if you are not well prepared. Work hard in your studies and cover all the questions and answers available in your syllabus in a stipulated time frame before you opt for this preference.
Coaching are the only chances of possible success in this option. One needs to be well prepared and appear the exam to impove the marks. However, note that you might get lower or higher marks than before or after.