I'm in the class teaching something. And I say something...
Don't be million miles away from this place. I'm going to teach you something. It is important for you/all of you/you all.
If I just say 'you', it's the whole classroom right? Do they all mean same?
Very good question !
You can certainly use "you" for plural in this context (or any other) and will not be misunderstood. This is the way that most teachers speak to their students as a group (at least when I was in school in the US). In formal English and in most dialects, the second person plural agrees with the second person singular.
"All of you" = strongly emphasizing that the thing you are about to say applies to everyone (even more strongly: "each and every one of you.") Unless you have a specific reason to do this, you shouldn't overuse "all of you." The default plural "you" means everyone too, it just doesn't emphasize it as much.
"You all" is a less strong way to say "all of you," or in some dialects it is the default 2nd person plural (see below).
Common informal 2nd person plurals in America are "you all," often contracted to "y'all," and "you guys." (I hope people from other continents will answer as well). Traditionally "y'all" was more associated with the South and African-American English, but it is spreading somewhat, perhaps due to perceived sexism in "you guys." In my native dialect "y'all" is used for the plural more often than not (but in formal speech, including the classroom setting you mentioned, "you" is used.) One of my earliest language memories is disliking "you" to mean "you all" from teachers, especially when the teacher was mad !
