Horace Greeley High School is a public, four-year secondary school serving students in grades 9–12 in Chappaqua, New York, United States. It is part of the Chappaqua Central School District.

It is consistently ranked among the top high schools in America. In 2015, it was listed as the No. 1 best public high school in the US by Best Colleges.

Greeley was ranked No. 46 nationally in the 2008 U.S. News & World Report rankings of "America's Best High Schools," and No. 7 among those with open enrollment. It currently offers 21 advanced placement courses.

Recent years have seen approximately one-tenth of graduating seniors recognized by the National Merit Scholarship committee. The class of 2004 included 25 National Merit semi-finalists, the class of 2005 had 16, and the class of 2007 had 22. The mean SAT and ACT scores among graduating seniors for the Class of 2022 were 1368 and 29, respectively.

The school offers several extracurricular programs. Its academic challenge team won the National Academic Championship in 2003, 2013, and 2023 and finished third in 2009 and 2010. Chip Beall, the organizer of the tournament, noted in 2007 that Greeley's team had "the most airline miles logged at the National Academic Association's expense", a nod to their placement in the final rounds of the tournament more times than any other team in the tournament's history.

The Horace Greeley Debate Team has sent debaters to the state competition every year since its inception in 2002.

The Madrigal Choir, a select group of students auditioned from the full chorus, has attended the Disney Honors festival in Orlando, Florida and has performed with other choirs at such venues as Carnegie Hall and Lincoln Center. In 2011, the Madrigal Choir received a gold award and came in second place at the Boston Heritage Festival.

Programs at Horace Greeley include the LIFE (Learning Independently From Experience) school, an alternative school for grades 11–12 located on campus, independent study, and senior project options. Language classes are offered in four foreign languages: Spanish, French, Latin, and Chinese. In the 2004–2005 school year, Ancient Greek was taught for the first time, as an independent study. Students have the opportunity to take Syracuse University Project Advance (SUPA) Forensic Science. Students may take Business Law for college credit from Mercy College.

The principal is Sandra Sepe, and there are four assistant principals.

The school is named for Horace Greeley, the editor of The New York Tribune who made his home in Chappaqua late in life. The school's main publication is The Greeley Voice.

Among the diverse offerings are varsity programs in baseball, basketball, bowling, field hockey, football, golf, ice hockey, lacrosse, skiing, soccer, softball, swimming and diving, tennis, track and field, cross country, volleyball, and wrestling.

The school's most successful sports team historically has been their boys' swim team. More recently, the boys' swim team has achieved even greater success, winning the NYSPHSAA New York State Swimming Championship for three years in a row from 2022-2024. Their success is due to the training of the year round swimmers on club teams in the area. The school's first state championship came in 2002 and was won by the boys' cross country team. In 2001 the school's football team finished with a record of 11–2, losing 22–15 to Rochester's Aquinas Institute in the New York State Class A State Championship game.

Greeley's boys' indoor track and field team won leagues, counties, and sectionals three years in a row, 2022, 2023, and 2024.

The school campus is made up of 11 buildings, all are named by letter. Buildings such as the Gym (A Building), Cafeteria (H Building), and Auditorium (B Building) are referred to as such and not by their letter name. Although the building letters span A through L, there is no I Building, for unknown reasons. Multiple athletic fields and a tennis court are also on campus. Horace Greeley High School originally opened in 1928 as part of the K-12 Horace Greeley School, which was located on the present-day site of Robert E. Bell Middle School. The high school's current campus opened in 1957.

L Building (L) is located towards the back of the high school, connected via a bridge to J building. Social Studies, Foreign Language, and English classes are all taught in L. There are 4 department offices located in L. L is split into two levels, called Upper L and Lower L. Prior to spring and summer 2020, L had normal size classrooms. When all Horace Greeley faculty and students were virtual in spring 2020, L was renovated to have open space and open classrooms. There are now "breakout rooms" and multiple common areas for students. The new classrooms were smaller and some had no doors, they are open to the hallway. The renovation of L coincided with the policy change that teachers would no longer have their own classroom. Instead, the teachers would be based in their respective department office.

Lowest L, as dubbed by the school's teachers, is located below L Building. The school's servers are located there as well as storage and a loading dock. It is connected to the interior of lower L through only one door and connected to the outside through one door and two garage doors. Lowest L is effectively a bunker.

In September 2001, the school received a flurry of press coverage over the football team attending a party where a stripper was hired for entertainment purposes. In 2015, drama teacher Christopher Schraufnagel resigned and was charged with the sexual abuse of three 15-year-old students. The crimes were alleged to have occurred between 2011 and 2015 on campus. The parents of three students subsequently filed a lawsuit against the school district. On November 7, 2016, Schraufnagel pled guilty. On December 21, 2017, Schraufnagel was classified as a level 3 Sex Offender. On December 30, 2022, Schraufnagel died of cancer.

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• School website
• School District website
• America's Best High Schools Ranking by U.S. News & World Report