Ha, I read “not likely” as being “unlikely”. I certainly cannot categorize William and Mary for this student. But I wouldn’t say it’s "unlikely ".
If this student is in the top 20%at Deerfield it is likely (using the word advisedly!) that they will end up at a T20 school. Which T20, I don’t know. And there’s always a chance that they won’t. But it’s more likely than not.