Stuart Kelly has been farewelled by hundreds of mourners at his alma mater,The King’s School, in Sydney, one week after his death.

The 18-year-old’s body was found last Monday, four years after his older brother died following a one-punch attack at Kings Cross.

The memorial service at the school’s Parramatta campus saw Prime Minister Malcolm Turnbull and NSW Premier Mike Baird pay their respects.

“For Ralph and Kathy (Kelly) to have lost their eldest son Thomas to a coward’s punch in 2012 was appalling. To have now lost their youngest son Stuart is beyond awful,” the school’s principal Tim Hawkes said in a statement.

“Although both boys had left the school when they died, they were still closely connected to the King’s ‘family’. Therefore, we are in deep grief.”

Stuart Kelly speaking after the sentencing of Kieran Loveridge in 2014. Photo: AAP

Stuart was 14 when his brother Thomas died after being randomly punched while walking with his girlfriend in Kings Cross in July 2012.

Following his brother’s death, Stuart went on to campaign heavily against alcohol-fuelled violence, and support the work of the Thomas Kelly Foundation.

The campaign, coupled with the death of one-punch attack victim Daniel Christie in 2013, eventually led to the controversial lockout laws being introduced by the NSW government the following year.

Principal Dr Hawkes said the school would continue to support the Kelly family in every way possible.