Yes, all your students can start at different levels. In the very first mission ‘Start Here’, after watching the introductory video and optionally tuning their guitar, simply have them take a skill test. Skills tests are designed to determine the skill level for each student. Upon successful completion of skill test for a level, all previous content in the syllabus will be unlocked automatically.
If a skill test is too hard, try an easier one. Conversely, if a skill test is too easy, try a harder one. This way, every student can find their level and progress from there.
Also, students who have a premium subscription can play any song from the "Songs" tab (to the right of the "Missions" tab) of any difficulty level.