I have re-watched The Force Awakens after going to see The Last Jedi. In TFA, Rey has a vision of Luke's Jedi school in ruins with the Knights of Ren standing in the ashes.
But in TLJ Luke gives more information on the attack (all a flash back):
Luke goes to kill Ben in his sleep sensing his growing darkness, but can't bring him self to do it. But Ben wakes up and defends himself (not with a red lightsaber), takes some students with him (the Knights of Ren), kills the rest and destroys the school thinking Luke is dead.
In the image it shows Ben in his Kylo gear and with his red lightsaber, how did he have this at Luke's school?