Please I would like to know the difference between to yell and to scream.
I am not qualified to answer your question. Nevertheless, I'd tell that I would use YELL for a cry or a shout that doesn't last long, except the case of a cheering up in unison.
A scream is LONG piercing cry. "His hilarity is like a scream from the crevasse." (Graham Green quoted) The plane flew through with screaming. "The place is very well and quiet and the children only scream in a low voice." (Lord Byron) Though, my Webster's dictionary tells that a scream may also be a sharp cry.
Here is a good English dictionary website to help you in your quest for better understanding of vocabulary!