A 32-year-old woman has been charged with murdering 18-year-old Alfie Stone.
Mr Stone died on Monday in hospital after receiving a stab wound in the chest near West Ruislip station.
Detectives announced late on Wednesday night they had charged Maxine Benson, of no fixed abode, with murder.
She will appear before Wimbledon magistrates on Thursday morning.
Benson was one of four people arrested so far over Mr Stone's death.
A 28-year-old man and a 29-year-old woman on suspicion of murder remained in custody at a west London police station, a Met spokesman said.
A 27-year-old man arrested on Monday has been bailed until mid-December while enquiries continue.
Mr Stone was a pupil at Pield Heath House, an academy for young people with special needs in Uxbridge.
He was believed to be with his elder brother and two teenage girls when he was fatally injured at about 5.35pm on Monday.
He is the 13th teenager to be fatally stabbed in the capital this year.
