For modern-era champions (2004 to present), the “Week 6 top-12” rule appears to hold without exception. 
For pre-2000s champions, many — like 1988 Kansas — appear to not have been top-12 by week 6; at least one (Kansas 1988) is documented at 18 that week.  
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For the last 20 years the eventual national champion has been ranked in the top 12 in the week 6 AP poll
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on December 8, 2025, 12:18:33, in reply to "