These are informal and casual statements. Both are used. There should be a comma after "Guys".
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These are informal and casual statements. Both are used.
There should be a comma after "Guys".
In both sentences, the word 'guys' is a vocative, ie a form of address, and should be followed by a comma.
Without further context, natural English is 'at'.
in is used more in an academic context where a level refers to a class.
Clive