"The 24-year-old is still under observation in the hospital," Munich police spokesman Gottfried Schlicht told The Local.
The student, a German citizen originally from Uzbekistan, sustained serious injuries to his skull and nose after the four girls and their male friend spat on the man and beat him brutally for annoying them with his telephone conversation while travelling on the U6 line.
Police said the boy was also treated for a mild hand injury after the student pulled a knife on the group during the attack.
The group comprised a 15-year-old girl from Venezuela, a 15-year-old girl from Kazakhstan, a 15-year-old girl from Ecuador, a 14-year-old girl from the Dominican Republic and a 15-year-old boy from Italy.
"Police are reviewing how they will press charges," Schlicht said. The police statement released Tuesday said the youths will likely face aggravated assault charges.
The train was stopped at the Nordfriedhof station where the injured parties were transported to the hospital and the attackers were arrested.
Their parents picked them up after police concluded their investigation.
A study released in June by the German Youth Institute in Munich reported that violence among girls in Germany has more than doubled since 1993, but Schlicht said he hasn't noticed a big problem.
"This was an isolated incident," he said.