From their student account, your students will be able to select which subject the would like to work on. However, if you would like them to work on a particular subject you will be able to restrict them to that subject for an hour.
To restrict a class to a particular subject you can:
1) Click on the subject at the top left of your dashboard
2) Tick the subject you would like to restrict your class to
3) Tick the option 'Restrict students to XXXX for an hour.'
4) Click Done to save
Note: All subjects excluding Mathematics are a subscription feature. Here is how you can order a subscription.
If you would like to restrict the subject an another one of your classes, here is how you can select the class.
In addition to restricting what subject your students will work on you can also restrict which activities they are able to access, here is how.