Hakim Sillah, 18, was attacked at around 4:30 p.m. in the youth offending service department of the Hillingdon Civic Center in Uxbridge, where police say a group of youngsters had been gathering at the venue when a fight broke out.
Sillah was taken to a west London hospital where he died an hour later, according to the BBC.
His mother reportedly dropped him off at the knife awareness session just 50 minutes before the attack, according to Sky News.
The suspect, an unidentified 17-year-old boy, was arrested on suspicion of murder and remains in police custody.
"We've made an arrest and we are not looking for anyone else at this time but equally we need witnesses to come forward or people who hold information," Metropolitan Police Detective Chief Inspector Noel McHugh told Sky.
The family for Sillah released a statement describing him as "a lovely lad who cared about his family."
"He loved looking after his little brother, who had been ill," they added. "The parents and family of Hakim would be grateful for any information from members of the public to help this investigation."
Police praised the courage of a third teenage boy who tried to break up the fight and suffered a non-life-threatening wound to the ear in the process, according to the BBC.
Prime Minister Boris Johnson, whose constituency is Uxbridge, tweeted about the incident on Thursday.
"Very sad news from Uxbridge last night. My thoughts are with the victims [sic] friends & family," he wrote.
The family for Tashan Daniel - who was stabbed to death at a Hillingdon subway station on Sept. 24 - plans to organize a march in London on Dec. 7 to raise awareness of knife violence, according to the BBC.