For your IGCSEs you wont have the time to write a story/essay with such a complex plot.
Aim to keep it simple, use good vocabulary and build on a relatively simple plot.
Simplicity is the name of the game - you have a word limit of like 500 words max for Paper 3 for the creative writing section.
Everything else (Summarys, letters etc) need to be one and a half page to two pages long max.
An Examiner isnt going to correct anything beyond the word limit and excessively long scripts are PENALISED.
I don't disagree with that. But many candidates can manage perfect English. What will make the difference between an A and an A* is your plot. Your creativity, your ability to keep your reader riveted.
And yup, like Dibbs said, using flashbacks surely gives a more mature impression.
The ideas, people have already overloaded you.
If you can, try to add something unexpected at the end.