It's understandable to feel nervous about this! The truth is, it depends on the type of essay you're writing. * For academic essays (IELTS, university papers, etc.), it's best to avoid most idioms because they are often informal. However, some "formal idioms" or "fixed expressions" (like "a case in point," "play a crucial role," or "in light of") can be acceptable in formal writing. * For opinion or creative essays, idioms might be more acceptable if they are relevant and used correctly. If you’re preparing for IELTS, you can safely use formal expressions, but avoid anything too casual.

I think it depends on what one means by 'idiom'. Some idioms can be more 'educated' and other more 'vulgar'. Also, you didn't mention what type of essay you're talking about. If you're writing a non-academic essay, for general readers, humor, entertainment, etc. you may want to use whatever idioms you like. Examples: Academic essay: King John was irate . . . . etc. Humorous essay: King John was mad as a wet hen; royally p'ssed off. fit be tied, etc. Bottom Line: if you're going to use idioms you need to use idioms appropriate to the type of writing and your reader.

I see no problem with using idioms in essays, but I'm not an academic. I'm curious to know what would be the examiner's excuse for frowning upon such things, because to me, it seems unfair.