William J. Brennan High School is the tenth public high school in the Northside Independent School District of San Antonio, Texas, United States. It is a four-year high school that opened in 2010 and graduated its first senior class in June 2012. During 2022–2023, Brennan High School had an enrollment of 3,142 students and a student to teacher ratio of 17.87. For the 2021–2022 school year, the school was given a "B" by the Texas Education Agency, with a distinction for Academic Achievement in Social Studies.

Brennan serves all areas west of Loop 1604 from just south of the Alamo Ranch neighborhoods west to the Medina County–Bexar County line near Texas State Highway 211 (Hill Country Parkway), extending south to near Marbach Road and its intersection at Grosenbacher Road. The far west areas of San Antonio, Texas, are experiencing rapid growth and Brennan alleviated overcrowded conditions at neighboring William Howard Taft High School and John Paul Stevens High School.

As with all the high schools in the Northside Independent School District, it was named after a United States Supreme Court Justice, William J. Brennan. Brennan was appointed by President Eisenhower in 1956 and retired in 1990. He died in 1997 at the age of 91.

The Brennan Bears compete in these sports -

• Baseball
• Basketball
• Cross Country
• Football
• Golf
• Soccer
• Softball
• Swimming and Diving
• Water Polo
• Tennis
• Track and Field
• Volleyball
• Marching Band

In the 2013–2014 school year, the football team advanced to the Class 4A Division 1 state championship football game and played Denton Guyer High School from Denton, Texas. The game was played in Arlington, Texas, at AT&T Stadium. Brennan fell short 31–14 and received the title of state runner-up with a 15–1 season record.

Brennan is fed by Luna, Briscoe, and Bernal middle schools, which are fed by elementary schools Behlau, Galm, Mireles, Hoffman, Langley, Lieck, Gamez, and Ott.

The Brennan Bear Band is under the direction of Thomas Kober with assistant directors Mark Wolocki and Tim Briones. The Bear Band has consistently received 1's at UIL marching contest since the opening of the school in 2010.

2012-2013 - The band placed first in several competitions and contests with their show, "The Fire Within." Among these were first place in Class 4A Northside ISD Marching Contest and first place in the USBands New Braunfel Showcase. At the Northside ISD Marching Contest, the percussion received the highest score of all the bands for "Percussion Captions." At the USBands New Braunfel Showcase, Brennan received first place for the categories of "Percussion, Visual, and Music".

2013-2014 - The band became the first 4A marching band in the Northside Independent School District to advance to the UIL state competition level. In the State of Texas (4A), the band placed 19th for their show, "Turning Points".

2014-2015 - The band competed in the 6A level with their show, "Mood Rings". The band received a first division in the UIL Region marching competition, advancing them to Area Prelims at Dub Faris Stadium in San Antonio. They performed first in Class 6A of the Area Marching Competition, where they fell short of a spot to Area Finals, placing 13th out of 20 bands competing for one of the ten Finals' spots.

2015-2016 - The band competed in the 6A division with their show, "Intergalactic". They received sweepstakes at the UIL Region Competition.

• George King (born 1994) – professional basketball player for Ironi Ness Ziona of the Israeli Basketball Premier League.
• Jordan Murphy (born 1997) – professional basketball player who last played for the Greensboro Swarm of the NBA G League.
• Derick Roberson – American football linebacker formerly of the Tennessee Titans of the National Football League and currently for the DC Defenders of the United Football League (UFL).

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