We don't know (yet)
What you've stated in your question is all that we know so far. As was mentioned in the film and novelization regarding the destruction of Luke's Jedi, some willingly left with him while the rest were killed. Considering they weren't all killed, we can at the very least surmise that Kylo/Snoke had some Force related use for them. Given that these students would have been training alongside him under Luke, it's not unreasonable to theorize that they would continue to follow him afterwards as the Knights of Ren. But as the knights themselves were not included as part of The Last Jedi, we still have no definitive answer.
It's worth mentioning that according to director Rian Johnson regarding the Knights of Ren:
We have a very full movie already there literally was just not room for another element...
If there was no room for expanding on their story, there certainly wasn't enough to dive into the missing trainees either. Whether there is a link between the two or not we'll have to wait until Episode IX to find out.