Check out this short video tutorial:
Here are step-by-step written instructions as well:
Step 1: Go into your classroom homepage and click on the button for the Move Students tab.
Step 2: Click on the student(s) you would like to move.
Step 3: Click the "Next" button on the bottom right corner of the pop-up screen.
Step 4: Select whether you would like to move them to another one of your classes, or another teacher's class that you have the code for.