You determine the structure of the contents for your Code Class yourself. However, everything is built up with increasing difficulty.
The most logical thing is to start with Scratch.
Some classes choose to handle strictly one challenge per meeting. So everyone stays at the same pace for a whole year. Differentiation takes place through the various extra tasks that are listed for each challenge. The fast workers are supposed to help the others and work together. This benefits the spirit and allows your pupils to practice their social skills. However, you can also choose to let everyone program at his or her own pace. You as a teacher, always check whether a challenge has been completed properly before the pupil is allowed to start the next challenge.
mBot or Microbit
You may notice that after a few months your pupils need something else: something tangible.
Then you can choose to switch to the chapters "mBot" or "Microbit". This does require a (modest) investment from your school or your parent council!
What you can also do (for example, if you only have a few mBots) is let a different duo get started every week. This way you alternate abstract coding with a real working robot.