A court ordered the girl to be flogged in front of her classmates following an assault on the school principal, according to the Saudi daily newspaper Al-Watan.
After the assault she was discovered to have concealed a mobile phone, breaking strict Saudi regulations banning the use of camera-equipped phones in girls' schools.
Al-Watan said a court in the northeastern Gulf port of Jubail had sentenced the girl to 90 lashes inside her school, followed by two months' detention.
The punishment is harsher than that dished out to some robbers and looters.
Saudi Arabia, a leading U.S. ally in the Middle East, is an absolute monarchy controlled by the Al-Saud ruling tribe, and lacks any legal code.
King Abdullah has promoted some social reforms since taking the throne in 2005 but diplomats say he is held back by religious clerics and princes.
Cinemas and music concerts are banned, while many restaurants and even some shopping centres cater to families only, especially on holidays.
Religious police roam streets to make sure no unrelated men and women mix.
The Saudi court system is exclusively controlled Wahahbi/Salafi clerics, and bans the employment of non-Salafi citizens, especially as judges.
Saudi Arabia is the world's leading country in the use of torture-by-flogging, public beheadings and publically crucifying condemned prisoners.
The country crucified two people in 2009, including one in the capital Riyadh during President Barak Obama's visit last April.
In September, twenty Saudi teenagers who ransacked shops and restaurants were publicly flogged.
Newspapers reported that the teenagers received at least 30 lashes each in a public square.
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