William Allen High School is one of two public high schools of the Allentown School District located in Allentown, Pennsylvania, in the United States.

The school, which is located at 126 North 17th Street, serves students from the western and southern parts of the city. Allentown's other public high school, Dieruff High School, serves students from the city's eastern and northern parts. Until Dieruff's opening in 1959, William Allen High School was known as Allentown High School.

With 3,705 students in the 2006-2007 school year, William Allen High School is the largest high school in the city of Allentown, the largest high school in the Lehigh Valley region of Pennsylvania, and the third largest high school in the state of Pennsylvania.

Allentown High School was first established in a building on South Penn Street in 1858. In 1869, the first commencement ceremony took place - three students graduated. A new high school was built in 1894 (which is now used as Central Elementary School) at Lumber and Turner Streets. In 1916, a new Allentown High School was opened at its current location on Seventeenth Street. The Annex and Little Palestra were built in 1930, and the Linden Street Wing was opened in 1957. The school was renamed William Allen High School in 1960. In 1975, a Library-Science Center was built on the site of the Little Palestra. In 2010 a new 9th grade center was built on the site where the St. Cloud Building was located at the corner of Linden and St. Cloud streets. It was then dedicated as the "Clifford S. Bartholomew Building".

Allen High School can refer to:

High school is a term used in parts of the English-speaking world to describe institutions which provide all or part of secondary education, but not always the highest years of basic education, which can be called a "secondary school" or "secondary college" or other terms, depending on the nation or region (see below: Usage by country). The phrase "high school" is often incorporated into the name of the related institutions.

The term (as "high school") originated in Scotland, with the world's oldest being Edinburgh's Royal High School from 1505. The Royal High School was used as a model for the first public high school in the United States, the English High School founded in Boston, Massachusetts in 1821. The precise stage of schooling provided by a high school differs from country to country, and may vary within the same jurisdiction. In all of New Zealand and Malaysia along with parts of Australia, Bangladesh and Canada, high school is synonymous with secondary school, and encompasses the entire secondary stage of education.

William Allen may refer to: