The helmet schools are (in no particular order) - Michigan, Notre Dame, Texas, Oklahoma, Alabama. Ohio State, Nebraska, Penn State and USC.
Just those nine and none other. You rank them by most wins, you get those nine. You rank them by winning percentage, you get those nine. No one else comes close in both categories. That's why it's only these nine.
Michigan hasn't won an MNC since 1997. So what? It's still these nine.
Texas and Nebraska have been bad. So what? Still these nine.
But what about Tennessee, Georgia, A&M and Michigan State? You don't become a helmet school by playing helmet schools or by having campuses in close proximity to other helmet schools. You're either a helmet school or not.
BYU is usually good. Won an MNC. Ain't never going to be a helmet school.
Clemson is good. Won a couple of MNCs. Ain't never going to be a helmet school.
There are, however, two additional tenuous helmets. That's not to say they have always been helmet schools like the other nine or will always be helmet schools but at this moment in time they sit at the table.
Additional tenuous helmet school # 1 is LSU. People have this vague idea they were once good back in the 1950s when Dennis Quaid played for them. And they've won several (or so it seems) MNCs in recent years. So we'll give them that despite their lack of blue blood.
Additional tenuous helmet school # 2 is the vague notion Floridian of success that gives a luster to Florida, Florida State and Miami. It's like none of them have the history to own a helmet but collectively they share one. So if a helmet represents 10 points, at any given time each of the three Florida schools may be splitting a 10 point pie. Some years, one of the three may have 8. Other years, two have three and one has four. But they squabble over that helmet and it's more than most non-helmet teams have.