kelvin0714

I also believe that your writing is quite good, despite some room for imporvement:

For grammatical accuracy, it is OK
careless grammatical mistake--> advices --> advice.
I or the police --> The police or I
mentor disorder--> mental disorder

I don't know whether they are typos, but you had better have no grammatical inaccuracy in DSE.

For the contents, you lacked some solid examples to illustrate your answer / argument.
In the part of "The best part of my job", you can cite some celebrities, like Obama, or Donald Trump, etc. In the part of "The biggest challenge I have ever faced in my job", you can cite some events (current affairs), such as Mong Kok Riots, Occupy Central Movement, for the sake of better impression.
You are reminded to ask a few questions in this writing since this is a "feature article". And you have to attract the interests of students. e.g. Rheotical questions, question tags.

For language used, you have shown a spectrum of vocabular items and sentence patterns, but you can make good use of other patterns like inversion, adj patterns.

This may be a level 4 writing. Add oil! I'm also a 2016 DSE candidate!