After the killing of a teacher in the Münsterland town of Ibbenbüren, school problems emerge as a possible motive for the suspected student.
The 17-year-old was expelled from school for one day after a series of conflicts with teachers on Tuesday, police and prosecutors said on Wednesday.
This was the result of the homicide commission's witness interviews.
On Tuesday afternoon, the 17-year-old is said to have attacked and killed his class teacher, who was alone in the classroom at the time.
He then dialed 911 himself and was arrested without resistance.
The youth has not yet commented on the crime.
The motive is unclear.
A public defender has been appointed for him.
The body of the 55-year-old teacher is to be autopsied this Wednesday.
A larger police presence arrived at the school after the crime on Tuesday.
In addition to several patrol cars, a rescue helicopter, an ambulance and an emergency chaplain also arrived.