According to the IHSA...

Loyola coach John Holecek.

...football classes, the five largest public schools in the state are (with enrollment):

1. Morton (8327)
2. Waukegan (4402)
3. Warren (4262)
4. New Trier (4170)
5. Lyons (3967)

And the five largest Catholic private schools are (with the 1.65 multiplier):

1. Loyola (3385)
2. Marist (2842)
3. Notre Dame (2745)
4. Brother Rice (2732)
5. St. Patrick (2300)

Now, if I'm doing the math right, here are their actual enrollments:

1. Loyola (2052)
2. Marist (1722)
3. Notre Dame (1664)
4. Brother Rice (1656)
5. St. Patrick (1394)

So if private schools were classified in order of their actual enrollments (and I'm not necessarily saying they should be), Loyola, as large as Highland Park (2052), would be in 7A. The other four would all be in 6A. Marist is a touch larger than Rock Island (1720); Notre Dame is a little bigger than Thornton (1657); Brother Rice is the same size as Lake Forest (1656); and St. Patrick is just shy of Springfield (1395).