Can I use a phrasal verb "to be on a roll" that means that something has a big success in the informal letter on my exam? No. "To be on a roll" means that someone has had multiple successes in a row. For example, if I pass 3 exams in a row, I could say, "I am on a roll". If you have only passed one exam, you wouldn't say "I am on a roll". Can I say that English songs are on a roll? No. One could say, "English songs *are blowing up* right now in my country". Does this make sense?

Perfectly acceptable in an email to a friend or in casual conversation. Not for university or college exams, unless the assignment is about using idioms/slang!

Use only simple formal language explaining your thought in the word. As much as simple you will try to do it will help you to not make things complected in English. Don't use slang in exam it will be definitely wrong there.

Can I use a phrasal verb "to be on a roll" that means that something has a big success in the informal letter on my exam? No. "To be on a roll" means that someone has had multiple successes in a row. For example, if I pass 3 exams in a row, I could say, "I am on a roll". If you have only passed one exam, you wouldn't say "I am on a roll". Can I say that English songs are on a roll? No. One could say, "English songs *are blowing up* right now in my country". Does this make sense?