Like Cordelia said, "you all" is usually used by people from the southern U.S. and it is not used often. In informal English it is more commonly shortened to "ya'll". It is the same as saying "you guys".

Thanks

"you all"means all of you. It is a regional expression most common in the southern part of the US. It would be better to say sentence 2. I think you say "Grade", but we say "form" here.

You all is simply a way of expressing the second person plural. If you are speaking to 2 or more people and you want to be clear that you are addressing all of them, you can say “you all.” I am not sure that I understand your question about “grade.”