The girl and the boy met in the park at around 5:30 last evening. Police said the two of them had a scuffle and he attacked her.
Police said the 18-year-old boy, Sarthak Kapoor, panicked after committing the act and went to his house. When the girl did not return home, her father approached the boy and his family to know the his daughter was.
After initial hesitation, the boy admitted to killing the girl, police said. He was soon arrested.
Police said after school, Sarthak Kapoor went on to pursue Bachelor in Computer Application (BCA) from an institute in Rohini.
The matter is being probed, police added.