The advice I generally give is you should only do creative writing for context if you have had previous writing experience (e.g. writing as a hobby) or you are very widely read. The way I see it, you have been 'trained' writing expository pieces for pretty much your entire high school life. Creative writing, on the other hand, is rarely taught throughout high school. Unless you have good reason, I would be hesitant to deviate from the style you are most familiar with.
Personally, I also see creative writing as something that shouldn't be written under time constraints. Creative writing is more than just having a unique idea. You need to also be able to convey that idea in a way that preserves its uniqueness and brilliance. At the start of the year I thought I had stumbled on such an idea. I thought of writing an exam piece which directly addressed the examiner reading it, similar to what Holden does in Catcher in the Rye. When I tried writing it, I realised I as just relying on the novelty of the idea and couldn't write a sustained piece. Creative writing simply wasn't my style and there was no way I could churn out a full blown creative piece in an exam setting.
This raises another danger to be aware of. It is very difficult to objectively judge your own work when you write creatively. Often students believe they have come up with an extremely unique idea when in fact the examiners have seen it done many times before. I'm sure there were hundreds of students who had thought of doing a similar thing to me and I'm glad I quickly scrapped the idea and stuck with what I was most comfortable with.
There is a common misconception that writing creatively is the only way to get a 9-10 or it is somehow easier to get a 9-10 writing creatively. As far as I am aware the examiners are not biased in favour of any particular style. I wrote exclusively in the expository style and was able to achieve consistent 9's and the occasional 10. The best thing for you to do is write in the style you are most familiar with. For most people this will be expository.
I'm not trying to dismiss creative writing or discourage you from writing in that style. Skilled writers who can produce amazing create pieces under pressure will be aptly rewarded. From my personal experience, however, I find creative writing risky and ill-suited to an exam situation.