A teacher lost two teeth and faced surgery to save his nose after being attacked by two drunken former pupils, Doncaster Crown Court was told.
The attack happened at Conisbrough Northcliffe School in Doncaster in September when the pair decided to go to the school and disrupt lessons.
Steven Thorpe and Matthew Godsmark hit and kicked teacher Michael Green when he confronted them on a playing field.
Thorpe was jailed for six months. Sentence on Godsmark was deferred.
At earlier hearings Thorpe had pleaded guilty to causing grievous bodily harm and Godsmark to affray.
Thorpe, who was on bail at the time, hit the teacher with a wooden pole and threw a full beer bottle in his face. Godsmark kicked the teacher when he was down.
Judge John Dowse told Thorpe he had been facing a long sentence but could not be given one because he was only 17 when the attack happened.
He was given six months on top of a sentence he is already serving for robbery.
Godsmark's sentence was deferred until October.
Both men will be subject to anti-social behaviour orders.