Once you have clicked on the student you can choose which display you want them to present on.
This will only give them access to the screens you choose. You can also assign different students to different screens.
If you would like unrestricted access to the screens, you can simply set the room to Open Access in Room Settings.
You can still revoke student access by pressing the Stop button. This will automatically stop streams if only one user is in control, or will show the Choose Display page if there are multiple users.