Luke was stabbed at Birkbeck School, North Somercotes in Lincolnshire earlier this month.
A 15-year-old boy, who can't be named for legal reasons, has been charged with his murder.
Luke's school was closed for the day to allow his close friends to pay tribute to him, and hundreds of people lined up outside the village church.
Luke's father Paul read part of an essay his son had written.
The essay said: "Things that make life worth living are the girls, especially the fit ones.
"There are also other things, like having fun, enjoying yourself and being happy.
"Things like that make life worth living. You only get to live once so you might as well enjoy life."
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