Some good examples here – you've argued clearly for having a wider range of subjects rather than a limited one. You've also decided to focus on the positive aspects of having many subjects and the negative sides of having a limited range, so I think your organization is good.   

The one thing I would suggest is using less of the passive. In para 1 you use "it is felt" and "this will be shown" 

EdHsiao said:

It is felt that education with broad subjects is likely to benefit student's career prospect in the long term. This will be shown by looking at how such form of education can bring more career options to 

I would suggest giving your opinion directly. Remember in IELTS Task 1 is factual, based on the data: Task 2 is opinion. Put as much of youself into it as you want! Don't be afraid to state your opinion – in fact this is required, and you will get marks for less common idiomatic and natural use of vocabulary (see the IELTS writing descriptors under Band 7, 8 and 9)!

• I feel that a broad education benefits the student's career prospects in the long term and will show how this can…

and in your conclusion:

• I really believe that having a wider range of subjects in high school will help the student in his or her career

So go on, give your opinon and don't be afraid to use "I"! 

One small point in the second paragraph:

EdHsiao said:

This example clearly shows, the merits 

no need for comma

Firstly, educating secondary students with broad subjects extends more career opportunities. Taking myself as an example, when I was in high school, I was not sure what my strengths and interests were, yet still I had to learn varying subjects such as mathematics and history. The foundation of mathematics built in college helped me progress well in my engineer career and the history learned in high school also played a key role in my later sales career and personal life as well. This example clearly shows, the merits of broad education in secondary schools can bring to students in the long term perspective.

On the other hand, skill-focus education excluding some subjects refrains students from further development towards professionals. For instance, footballer training course in some college of US may focus on preparing students into professional football players and ignore other subjects. With high incidents of injury and short life of football career, a retired football player often find difficulties in retraining themselves into other professions due to lack of fundamental knowledge in high school. This outlines that one's career can be limited by excluding some subjects in secondary colleges.

In summary, as above shows how more careers perspective can result from education with broad subjects and the adverse effect of excluding some subjects leads to limited career outlook. It is thus believed that high school education with broad spectrum of subjects will benefit students in their long term career.

You can find a sample essay here: http://staging5.writefix.com/newessays/?p=546. But trying writing your answer first! Use the timers above to help you. You have 40 minutes in the IELTS exam for Task 2, the argument essay.