From a different view point to many expressed here, sitting a ‘difficult’ child with a well behaved one may not be so as to use the child as unpaid labour (....), it may be so that the children can learn from each other. Goodness, some children have such difficult upbringings and don’t learn all social norms at home. It won’t help them- or society- to be sat in a corner away from all the well brought up children. At my school we always mix social groups, behaviour and ability.
Polite and bright children are pretty popular and highly regarded at my school and often children labelled troublemakers learn to apply themselves from working alongside these peers. And they learn that it can be cool to be smart and do work!
As for the group work being dismissed as mumbo jumbo... well. Not sure where to start! Speaking and listening skills (especially important in schools with lots of pupils with EAL) are developed, understanding of how they are learning (metacognition), questioning each others ideas, bouncing ideas off each others, like adults do in grown up work. Developing empathy (god forbid!).
Mixed ability groupings give everyone the opportunity to strive and do their best, not have a fixed ability mind set (I’m not good at maths, etc). Teachers obviously differentiate so some children are more supported and some are challenged further, but essentially it creates a great classroom dynamic where no one feels excluded from the best learning or like a failure aged 7 or whatever. Also, lots of research supports it as good practice in a modern classroom.
But if you want rote learning or good old fashioned schooling maybe not for you!