If a school doesn’t rank, Yale would not know the highest GPA in the class. Yale would only be able to see the GPAs of students from said HS who applied to Yale.
Most HSs in the US don’t rank…around 1/3 of HSs still rank, many/most are in TX. Most HS school profiles don’t show highest GPA (if they do, it would be from the prior year’s graduating class, not the current class applying with their six-semester transcripts. Many HS profiles only show median or mid 50% GPAs, again, from the prior year’s class.)
Regardless, I do expect Yale looks at relative academic performance in the cohort of applicants from a given HS…but they will not have the full picture of the academic performance of that entire HS class.