What if you're addressing part of the class, though? Like "y'all in the back, you need to get back to your work".

"You in the back" has the same level of specificity. Other options include (again) naming names or calling out a more specific location "You in the back row".

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madcaptenor an hour ago | parent [-]

No, because "you in the back" could refer to just one person in the back, instead of several. So "y'all in the back" is more specific. (Of course names are an option in this context.)

	▲	dfxm12 38 minutes ago \| parent [-]
		(Of course names are an option in this context.) Yes, this is a case where you aren't forced to use "you" ambiguously in that context. No, because "you in the back" could refer to just one person in the back, instead of several. If you meant to address one person, you'd have said that one person's name, instead of voluntarily introducing ambiguity to the situation. Context & body language also makes this obvious. If you meant one person, you'd be making eye contact with one person instead of a group of people, etc. Students also know if they're paying attention or not. "The back" is not a specific area.