US human rights group the American Civil Liberties Union (ACLU) has threatened to sue a Pennsylvania school board which banned students from forming a gay-straight alliance.

Last month the Chambersburg Area School Board voted 5 to 4 against a motion to allow a group of students to form the GSA club.

The ACLU has written a letter to the School Board, saying the decision to ban the club was in violation of US federal law, and has given the board until Friday to reverse its decision or face a lawsuit, reports the Associated Press.

Without the School Board’s approval a gay straight alliance club would not be able to use the school’s PA system, put up posters, advertising, or hold any events.

Meanwhile the Lake County School Board narrowly voted on Monday to allow extracurricular clubs, including a gay-straight alliance, after the ACLU put pressure on it to either allow the GSA or ban all clubs. 

The board voted 3 to 2 to allow the clubs.

The contentious issue began after a group of students at Carver Middle School in Leesburg attempted to start a gay-straight alliance club.

After the school initially declined to allow the club, the American Civil Liberties Union put pressure on the Lake County School Board to either allow the club, or to ban all similar extracurricular clubs.

A plane had reportedly been flown over the school board building in Lake County on Monday, urging it to allow all clubs.