Help draw out their ideas
Children could produce a "Wanted" poster for a shape that has gone missing from class and the focus could be to explain its properties. This can produce an interesting range of responses from groups, especially if the shape chosen is a parallelogram.
For example, if every group draws and describes a parallelogram as a "pushed over rectangle", you know you have work to do since a parallelogram could also refer to a rectangle, rhombus or square.
Another idea is to challenge children to produce a "How to" poster. They could describe how to add fractions with unlike denominators, how to subtract using a counting-up method or how to draw a reflex angle.
You can then run a poster conference where children "publish" their posters for other groups to look at and offer feedback using two stars and a wish