Thank you for your question. The first thing I recommend is that you review the video explanation for the setup of this game in the LSATMax app. The rules of this game establish that W is 6th and does confess, and that S is 7th and does not confess. The rules also establish that Z must confess before X and V and further that Z must be first or second.
Notice the question stem on this question says, "If Z was the second suspect to confess . . . " Well, if Z is the SECOND suspect who DID CONFESS, that means that Z is second, and whoever is first ALSO confessed. This is why, on this question, it must be true that Y (who goes 1st) confesses.
Hope this helps! Please let us know if you have any additional questions.