This has been updated for 2012-2014 here:
http://dallas-area-schools.blogspot.com/2012/02/2012-2014-uil-reclassification-and.html
The University Interscholastic League (UIL) announced its realignment for 2010-2012 on Feb. 1, 2010. Mostly this pertains to which districts high schools will be in for football, basketball, and other sports, but it also determines which "class" the high schools will be in for the next two years - 5A, 4A, 3A, 2A or A - which is determined by student body population. You can get a general idea of the size of a high school by which class it is in, with 5A being the largest high schools in the area.
For the first time since 1990 the cutoff dividing 5A and 4A has gone down causing some schools to move up.
UIL cutoffs for 2010-2012
Class 5A - 2,065 and higher (student population)
Class 4A - 990-2,064
Class 3A - 430-989
Class 2A - 200-429
Class A - 199 students and lower (both 11 man and 6 man football teams)
5A schools
District 4 - 5A
Arlington High
Arlington Bowie
Arlington Sam Houston
Arlington Martin
Fort Worth Paschal
North Crowley
Weatherford
District 5 - 5A
Cedar Hill
Duncanville
Grand Prairie
Mansfield
Mansfield Legacy (moved up from 4A)
Mansfield Timberview (moved up from 4A)
Midlothian (moved up from 4A)
South Grand Prairie
District 6 - 5A
Colleyville Heritage
Euless Trinity
Grapevine
Haltom
Hurst LD Bell
Irving
Irving MacArthur
Irving Nimitz
Richland
District 7 - 5A
Coppell
Denton Guyer (moved up from 4A)
Keller
Keller Central
Keller Fossil Ridge
Justin Northwest
Saginaw (moved up from 4A)
Soutlake Carroll
District 8 - 5A
Allen
Lewisville
Flower Mound
Flower Mound Marcus
Hebron
Plano
Plano East
Plano West
District 9 - 5A
Jesuit (private school)
Samuell (moved up from 4A)
Skyline
Sunset
WT White
Richardson
Richardson Berkner
Lake Highlands
District 10 - 5A
Garland
Garland Lakeview
Garland Naaman Forest
Rowlett
Sachse
McKinney Boyd
North Garland
South Garland
District 11 - 5A
DeSoto
Longview (moved up from 4A)
Mesquite
Mesquite Horn
North Mesquite
Tyler Lee
4A schools
District 4 - 4A
Denton
Denton Ryan
Lake Dallas
The Colony
Little Elm
Wichita Falls
Wichita Falls Rider
District 5 - 4A
Aledo
Azle
Birdville
Fort Worth Boswell
Keller Timber Creek (up from 3A, new to football)
Northwest Byron Nelson (up from 3A, opened 2009, new to football)
Springtown
Fort Worth Brewer
District 6 - 4A
Fort Worth Heights
Fort Worth Carter-Riverside (up from 3A)
Fort Worth Dunbar (up from 3A, by request, not size)
Fort Worth Eastern Hills
Fort Worth North Side
Fort Worth South Hills
Fort Worth Southwest
Fort Worth Trimble Tech
Fort Worth Western Hills
Fort Worth Wyatt
District 7 - 4A
Arlington Seguin
Burleson (down from 5A)
Cleburne
Crowley
Everman
Granbury
Joshua
Mansfield Summit (down from 5A)
District 9 - 4A
Carrollton Creekview (moved down from 5A)
Carrollton Newman Smith
Carrollton RL Turner
Frisco
Frisco Centennial
Frisco Heritage (moved up from 3A, new to football)
Frisco Liberty
Frisco Wakeland
District 10 - 4A
Highland Park
McKinney
McKinney North
Richardson Pearce
Rockwall
Rockwall-Heath
Wylie (moved down from 5A)
Wylie East (new to football, school still only has 3 grades)
District 11 - 4A (all Dallas ISD schools)
Adamson
Carter
Kimball
Molina (down from 5A)
Pinkston
Seagoville
South Oak Cliff
North Dallas
District 12 - 4A (all Dallas ISD schools)
Bryan Adams
Conrad
Hillcrest
Thomas Jefferson
Lincoln
Spruce (has not had athletics for last 2 years for scholastic reasons)
Woodrow Wilson
District 13 - 4A
Denison
Greenville
Mount Pleasant
Royse City
Sherman
Sulphur Springs
Texarkana Texas
District 15 - 4A
Corsicana
Ennis
Forney
Lancaster
Mesquite Poteet
Red Oak
Terrell
Waxahachie
West Mesquite
3A Schools
District 8 - 3A
Alvardo
Glen Rose
Hillsboro
Kennedale
Venus
West
District 9 - 3A
Argyle
Aubrey (moved up from 2A)
Gainsville
Pilot Point (moved up from 2A)
Sanger
Whitesboro
District 10 - 3A
Anna
Bonham
Celina
Commerce
Van Alstyne
District 11 - 3A
Farmersville
Frisco Lone Star (new school)
Lovejoy
Nevada Community
Princeton
Prosper
District 12 - 3A
Carrollton Ranchview
Dallas Madison
Dallas Roosevelt
A. Maceo Smith (down from 4A)
Fort Worth Polytechnic (down from 4A)
Fort Worth Hill-Jarvis
District 13 - 3A
Crandall
Ferris
Kemp
Kaufman
Mabank (down from 4A)
North Forney (new to football)
District 14 - 3A
Canton
Emory Rains
Lindale (down from 4A)
Quinlan Ford
Van
Wills Point