The PST is part of the standard recruiting process of McKinsey, so if it enforced in US, you would definitely have to do it, even though you are from a target school.
However, in a few countries (I don't know if it is the same in US as well) the PST has been substituted with a much easier online test. Maybe that could be the reason why you haven't heard about it.
The test is made of a part of "true/false/can't say with the provided data" part (basically a few questions with 5 lines each and a statement that you have to say to be true/false/not inferable) and a math part with graph understanding, currency changes, tables, ...
If you are not sure about the presence of PST, I would definitely suggest you to directly contact the HR and directly ask them. Don't be shy to contact the HR in advance, they are always happy to answer questions from their candidates and see them better prepared to become possible consultants. It would be seen as a sign of interest and much appreciated from their side :)