The contracted, and very informal, American English expression “y'all” is indeed plural, just as “you all” is. Both have the same meaning: “every one of you”.
Compare: Have you passed? (the subject could be singular or plural) and Have you all passed? (the subject is plural).
The OP's example is marginally ambiguous, (see comments below) the teacher could be telling the individual student that they will be awarded two or more As for their assignments but this situation is highly unlikely. To avoid any possible misinterpretation, we can use the expression I mentioned earlier "every one of you"
I will give every one of you [straight] A's"
I will give you, [student's name], all A's