So have they been asked to write their own suicide note? (stupidly crass/of no educational merit) or as Lady Macbeth's which is writing in role having committed regicide & being culpable of other murderous deeds?
If the latter the task shows understanding of character/theme & recall of quotation which is necessary for the exam. However, the task needs tweaking. Macbeth writes to his wife earlier in the play and now presumably the idea is she - cut off from him - writes back. No need to call it a suicide note although, if set having read Act 4 and you know the play, it would be. But hardly triggering if writing in role unless you've got regicide, a host of other nefarious murders & a bit a baby brain dashing under your belt.
Shakespeare, the texts at GCSE & much of adult literature deals with frightening and socially taboo subjects as a way of making such subjects explainable & less frightening.
I can't get too aerated by this [shrug]. I can remember a Lutterworth exam invigilator getting the now Poet Laureate's 'Education for Leisure' banned as she thought it glorified knife crime. The stupid bint didn't understand it did exactly the opposite, as 300,000 GCSE students could attest.