In the book, Lavender's death isn't explicitly confirmed, nor have there been any statements from JKR confirming her fate. For several months (until December 2015) her "Fact-File" on the new Pottermore site originally stated that she was "Presumed Dead, 2 May 1998" however this has been amended to remove that statement (without explanation).
In the film, the scene where she's killed is much more definitive and there have been follow-up interviews that absolutely confirm her death at the hands of the werewolf, Fenrir Greyback
In the final Potter book, Lavender's last mention is that she was "feebly stirring" (see below) however her injuries seem extremely severe, she isn't mentioned again, she isn't on Platform 9¾ in the final chapter, nor does she appear as a character in Harry Potter and the Cursed Child.
Harry, Ron and Hermione sped down the marble staircase: glass
shattered to their left and the Slytherin hourglass that had recorded
house points spilled its emeralds everywhere, so that people slipped
and staggered as they ran. Two bodies fell from the balcony overhead
as they reached the ground and a grey blur that Harry took for an
animal sped four-legged across the hall to sink its teeth into one of
‘NO!’ shrieked Hermione, and with a deafening blast from her wand
Fenrir Greyback was thrown backwards from the feebly stirring body of
Lavender Brown. He hit the marble banisters and struggled to return to
his feet. Then, with a bright white flash and a crack, a crystal ball
fell on the top of his head and he crumpled to the ground and did not
In an interview to launch the UK DVD, the actress who played Lavender Brown (Jessie Cave) stated that we do indeed see her character die at the end of Deathly Hallows Pt II.
Q : What was the last scene you filmed on the very last film?
Jessie Cave : Mine was my death scene. Which is quite fitting, really.
This is also confirmed in the tie-in book Harry Potter Page to Screen: The Complete Filmmaking Journey
Jessie Cave was also happy to come back, even knowing that Lavender
Brown wasn't going to survive the final film. "She's not very lucky,"
the actress understates. "She doesn't meet a nice ending. She gets
eaten by Fenrir Greyback."
Cave recalls her death scene, which took several nights of shooting.
"I had the most amazing prosthetic of a scar that looked so scary, it
looked like I was actually mauled to death."