The 40-year-old accused has been identified as Baldev Singh, who had been working as a sweeper in the school in Vikaspuri for the last 15 years, police said.
A native of Bilaspur in Uttar Pradesh who had migrated to Delhi few years back, Baldev had been engaged with the school through a contractor and used to do menial jobs, said a senior police officer. He allegedly took the girl inside the school's washroom where he molested her on September 16.
He has been booked under sections 354 IPC (assault or criminal force to woman with intent to outrage her modesty) and the relevant section of the Protection of Children from Sexual Offences Act, DCP (West) Pushpendra Kumar said.
The parents of children studying in the school, including the victim's parents, had been protesting against school authorities for not taking swift action in the matter.
The child had informed her parents on September 16 that a mustachioed man allegedly took her inside the washroom of the institute around afternoon and molested her.