You need to have the Trainer role, and be assigned to the classroom as a trainer.
The Classrooms 
- Click the In Session or Upcoming tab, and then click the classroom name.
- Click the Labs tab, and then click the name of the lab you want to practice.
- If the lab is not active (showing a green dot), click Activate Lab.
- Click Open Lab.
The lab opens to the lab manual page. This is the same view of the lab that students in the class will have. Read the manual and then click the Start Lab button.
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