Lesson Activities are locked by default and unlocked by the teacher. Meaning students cannot open activities until you have unlock them.
An activity is automatically unlocked when opened in Teach Mode with the 
Classroom Setup Button.
To unlock/lock an activity, simply click on the 
lock icon in the lower right corner of an activity on the Lesson Plan.
Note: Locking an activity that is currently unlocked will immediately disable further changes for students who have the activity open.