Hello Adios (por así decirlo),
Unfortunately the sentence isn't entirely grammatical or idiomatic as it stands. It should be:
1. The LPI examiners are aware that writing perfectly idiomatic English is not an easy task for students for whom English is a second or third language.
Your suggested version (with "whose") is ingenious and not incorrect; though it would probably sound a little awkward to most native speakers.
All the best,