A student needs to be enrolled (associated) in an organization for testing. You can transfer student's enrollment from one organization to another through request and approval process as explained on this and
To request an enrollment transfer, follow these steps:
From Setup > Work Requests, click the Select Tasks drop-down and select Request / Edit Enrollment Transfer. Click Start.
To search for the student to be transferred, enter student details, and click Search.
Under Change Enrollment To select the organization where the student needs to be transferred. Click Send Request.
The student will be transferred once the transfer request is approved, as described in 3.1 NEW Authorization for Enrollment Transfer.
For enrollment transfer approval, see 3.1 NEW Authorization for Enrollment Transfer.
To understand how to manage student enrollments, see Manage Student Enrollments in an Organization.