Is "till" in the following sentence valid?
You must write in formal style till the end of this exercise.
I don't think it's correct, but I can't really say why. I read a question on ELU about this, https://english.stackexchange.com/questions/6989/what-is-the-difference-between-till-and-until , but none of those answers would say you can't use "till" here.