The 18-year-old girl was severely injured after he kicked, choked and beat her in a Hamburg school yard on Thursday. He stopped only after several teachers at the Borgfeld district school intervened, police said. The girl was hospitalised with a lower leg fracture and contusions to her head and upper body.
The boy was arrested on suspicion of dangerous bodily injury.
In May the suspect was released from police custody after being detained for six months on remand. The manslaughter trial against him and another boy had to be rescheduled because the judge's return from a Spanish holiday was delayed by the volcanic ash cloud that crippled air traffic over Europe for several days in May.
The two boys had been accused brutally beating a 44-year-old man at an S-Bahn commuter train station after he refused to give them €0.20. The man died four weeks later of a severe head injury.
Their victim's family and the wider German public were outraged by their release.