In formal writing essay and also informal writing essay, If ever possible I always try to mix up some idioms because it looks good to the readers and aside from that, it makes some good points to the writer if you did that but of course It really depends on the teacher but in my opinion, put idioms and it will look good and humorous ^^

It will depend upon the teacher/school rules. I teach Junior High language, and let me share our rules. In writing an informal essay, using idioms can "color" the essay and make it more interesting, and I my students at school to use a couple but not very many. In a formal essay, an idiom may be used in quoting someone or when specifically trying to prove a point. Poems can be tricky. If a short poem is being used at the end to "tie" the essay together with one more example, it may be printed out. A longer poem (3 stanzas or more) is unacceptable, but the title may be referred to in italics. I hope this helps.