From the Admin or Teacher app, tap on the Roster icon at the top of the home screen. Tap the three dots in the top right corner and then tap "Transition students". Checkmarks will appear on the right hand side of each students name. Tap to check the students that you'd like to transition into a classroom. All of the students that you've checked off do not need to be in the same classroom currently but they should be moving into the same classroom.
Once all transitioning students are checked off, tap the "Finish" button at the bottom of the screen. All active classrooms will appear. Tap the classroom you'd like to transition students into. A success banner will appear at the top of the screen and the student's current classroom will appear to the right of their originial classroom in the Roster.