There are tons of theories on homogeneous and heterogeneous grouping. Who really knows which one is better? What I do know is the application of grouping students is situational. It really depends on your students and what works for them. I've been grouping students both ways, but I cannot tell which is more effective. Today, I did a goal orientated activity with my class. I grouped a few students with their friends and warned them that if they use their time to chat with their friends, they would miss out on the prize. What I found was actually shocking. They worked really well together. They were one of the better groups that completed their task correctly and faster than the others. I've grouped them with each other as well as separated them before, but they would always end up chatting with each other. I think this experience goes to show how well tangible rewards really motivate students.
Currently in my fifth year of teaching Pre-K at a early childhood elementary school.