Leader should say something like: "If you need assistance or don't understand, please see me before/after class (or during break, etc)." And clarify by saying "You may not be able to finish this in class (or this weekend/retreat) and may need to finish on your own at home."
I'm a slow quilter/piecer/sewer and never seem to finish during class, so I reach out to the instructor to see if I can at least do the cutting prior to class.