Warning: spoilers ahead for Star Wars: The Last Jedi!
When Rey first picks up Luke's original blue lightsaber in The Force Awakens, visions of past events slam through her — including the seeming destruction of the Jedi academy Luke Skywalker attempts to set up after Return of the Jedi. Though we first assume Kylo Ren attacked the temple and slaughtered all the young Jedi-in-training, The Last Jedi shows us two points of view of that fateful night.
Via Force visions connecting them, Kylo Ren tells Rey that he woke one night to Luke standing over him with his lightsaber, and Kylo, then known as Ben Solo, reacted defensively. When Rey confronts Luke about it, he admits it's true — to a degree. While he did momentarily give in to his fear of Ben's strong connection to the Force, he realised just as Ben woke how wrong he was. Unfortunately, by the time he emerged from the rubble after Ben fled, the temple was going down in flames and, as he tells Rey, many of his pupils were dead . . . and the others had disappeared.
OK, hold up. Others disappeared? That implies that some of them survived, right? So where did they go?
The most probable theory, of course, is that when Ben Solo ended up fleeing from his apparently murderous uncle, some of the Force-inclined younglings left the temple with him after a battle. Whether Supreme Leader Snoke drew Ben Solo to him or found them through the Force, Ben and the remainder of the apprentices must have finished their training with Supreme Leader Snoke . . . and formed the newly minted Knights of Ren. As it stands, The Force Awakens refers to Kylo Ren as "master of the Knights of Ren," and The Last Jedi references his raw power frequently, implying the others, wherever they are now, aren't strong enough for Snoke to keep them nearby (or for them to matter yet).
Now that Kylo Ren has declared himself supreme leader after killing Snoke and the rest of the Praetorian Guards with Rey's help, though, we can assume he'll assemble his ranks in Episode IX. When Rey and the remainder of the Resistance emerge next to challenge the First Order, they're in for a surprise regiment of Luke's former padawans turned dark. Hopefully they find more Force-sensitive kids like the one in Canto Bight to fight for the Resistance and back them up!