I included all the relevant basketball schools, they are in order of enrollment:

At the same time, the Board approved a revised “Playing-Up” policy that allows a school to play in a higher classification, if desired.  A school wishing to play up must make a two-year commitment to the higher class in all sports run under the same system. 

Class 4A

Joliet
Curie
Stevenson
Neuqua Valley
New Trier
Warren
Waukegan
Brother Rice
Lyons
Sandburg
Lockport
L-W East
Leyden
Waubonsie Valley
Bloom
Bolingbrook
Loyola
St. Patrick
West Aurora
Downers South
Evanston
Oak Park
Young
Lake Park
Marist
Homewood-Flossmoor
L-W Central
East Aurora
Proviso West
Zion-Benton
Glenbrook South
Schaumburg
Glenbard East
Mount Carmel
Belleville East
Andrew
Plainfield Central
Conant
Thornton
Hisndale Central
St. Rita
Stagg
Niles West
Von Steuben
York
Wheaton South
Larkin
Farragut
Thornwood
O'Fallon
Mundelein
Hoffman Estates
Lincoln Park
Wheaton North
Jacobs
Elgin
Quincy
St. Ignatius
Alton
Proviso East
Moline
East St. Louis
Pekin
Benet
Glenbrook North
Glenbard West
Bogan
St. Laurence
Rockford Boylan
Morgan Park
Vocational
Hyde Park
Hersey
Rockford Jefferson
Julian
Libertyville
Bradley
Fenwick
Thornridge
Highland Park
East Moline
Batavia
Shepard
Argo
Eisenhower
Oak Lawn
Geneva
Romeoville
Deerfield
Reavis
Providence
TF South
Lake Forest
Foreman
South Shore
Prairie Ridge
Northside Prep
Richards
TF North
Hubbard

Class 3A

Simeon
Dunbar
Rock Island
Danville
Bloomington
Washington
Kenwood
Senn
Joliet Catholic
Rich Central
Oak Forest
Galesburg
King
Corliss
Hope
R-B
Kankakee
Prosser
Freeport
Payton
Peoria Richwoods
Hillcrest
Champaign Central
Lemont
Rich East
Mt. Vernon
Decatur MacArthur
Harlan
Springfield Lanphier
St. Joseph
Brooks
Robeson
Harper
Rich South
Bremen
Decatur Eisenhower
Austin
Carbondale
Marshall
Fenger
Sycamore
Wells
Bartonville Limestone
Washington, IL
Peoria Central
Yorkville
Carver
Morris
Crane
Manley
Peoria Woodruff
Gordon Tech
Lincoln
Pontiac
Johnsburg
Westinghouse
Mahomet-Seymour
Englewood
Leo
Collins
Hales
Phillips
Burlington Central
Bronzeville
Bishop McNamara
Rok Falls
Driscoll