For changing the teacher for a set of students permanently see Permanently Change Teacher in Bulk.
To reallocate students, navigate to Mark Rolls (Home Menu > Common Tasks > Roll-Marking Tuition > Mark Rolls) and find the teacher's relevant rolls. You can then change the teacher for the selected student or the found set under More.
This will change the teacher for those students for the viewed weeks rolls only. To run this process for just some students on the roll, first omit the unaffected students.