To ensure that your students work on the set Challenge from your teacher account you will be able to restrict them to only work on the Challenge for an hour.
If they complete the Challenge before the hour is up they will go back to work on their skills in Training mode.
To restrict your students:
1) First select which class you would like to restrict (if you are not already on it), here is how you can switch classes.
2) Click on Set work at the top
3) Select Live controls
4) Click on RESTRICT in the Focus Work box
5) Tick Challenges and competitions to restrict your students to only work on that activity for an hour. (You can select more then one activity to restrict your students to).
6) Click Restrict at the bottom right to confirm
Now when your students access their accounts they will only be able to work on the activities you have restricted them to.