First, the bare meaning of the words:
You might think you will never be a top student.
This is not necessarily so.
Three parts to understand the second sentence:This, not so and necessarily.
This is the proposition that you will never be a top student.
If we just said This is not so we contradict the proposition, we assert you will be a top student.
The addition of necessarily allows for uncertainty, ** This is possibly not so** or to state it positively it is possible that you might be a top student. Note that there is no explanation of this positive assertion, just a statement of opinion.
So, your statement 2). is not quite right, we are talking about possibilities not certainty.
Your statement 1). follows from the reasoning above. We assert that you may possibly be a top student, therefore we believe that your current thinking is wrong.
A paraphrase of the sentence is
Everyone, and I mean everyone, will be there.
As the Oxford dictionary (definition 1.1) says:
Used with repetition of certain words to give emphasis.
‘nobody, but nobody, was going to stop her’
This can also be paraphrased as
Nobody, and I mean nobody, was going to stop her.
Best Answer
I think it might be helpful to think of the dictionary's definition as:
"You should worry" would be said in only two contexts in English.
The speaker is confirming/adding new information, or
The speaker is contradicting.
The speaker will be Person A and the listener will be Person B.
In context 1, Person B is already worried about something. Person A states, "You should be worried", meaning that he agrees with Person B.
In context 2, Person B is not worried about something. Person A states, "You should be worried", meaning that he believes Person B is wrong and not worrying enough.
As an aside, English has a verb "to mean" that would make your sentence sound more your question more natural sounding.