How can I move students from one group to another

Hello Zoya,
Please follow these steps to manage your students:
1) Go to My Students > All students.
2) Select/tick the students you want to move to another group.
3) Choose the action "Remove from all groups" and click "Apply the selected items."
4) Next, select the same students again and choose the action "Add to groups," then click "Apply the selected items." Select the group and click "Apply."
P.S. This process is similar to what you did with a single student earlier when you had to remove a kid from an adult group.
I hope it helps.
Ruta