If the timeslot already has been booked you cannot make them unavailable. 

You would need to firstly print the teachers scheduled then individually go and remove that times for each student. 

Go to Manage Bookings > Find the student > change the session or remove.  

If bookings have not been made, go to Teacher search and select teacher from drop down and manage unavailability’s. 

Ideally, the actual teacher can do this too.