People often ask me whether its ok to use phrasal verbs in the IELTS writing and speaking tests.
My answer is yes; phrasal verbs are generally fine. In Fridays " IELTS Speaking Part 2: family celebration " I used the phrasal verb to grow up, and the multi-word verbs to take place and to make sure. In " IELTS Writing Task 2: private schools essay " essay, I used the phrasal verb to result in.
It isnt true that all phrasal verbs are informal. Read this article for a full explanation.