@littlek,
Well, if it was me, k, I'd request that
both of you speak to the group together. Having worked out a really good joint approach with your supervisor first, of course.
Up to you, of course.
But from the students' perspective .. they'd know that you're a
united front on this & that they can't play "divide & conquer" games anymore.
From your perspective: You'll know what has actually been
said to the group & how the group responds, first hand. (This is what's called called watching your back.
) And also it also doesn't look (to the students & their parents) as though your supervisor has stepped in to intervene,
over your head, in a situation that you've not handling well. It doesn't undermine your authority with the group, in other words.
This is not at all an unusual approach to such situations, in my experience.