Dear Humayun,
Please check the following links covering your questions.

http://www.cssforum.com.pk/thread4.html
http://www.cssforum.com.pk/thread2.html

In addition to this... in my humble opinion there shouldn't be any headings in the essay. Provide an outline before starting essay so that the examiner has an idea of what you're going to present. Do check what is required.. because if the essay asks you 2 things (e.g. Pakistan's socio-economical problems and their effects then) you have to cover both of the things in your essay. If you leave out on of them, you won't score good. Try to provide all pros and cons and then explain your opinion and provide a solution. Remember the definition of essay is: a short analytical, descriptive, or interpretive piece of literary or journalistic prose dealing with a particular topic, especially from a personal and unsystematic viewpoint... means you have to analyse or describe the topic and give your personal viewpoint.
I hope you can find more on the threads above.
For more details about common mistakes or examiners' reports, check out some A' Level's Essay book.