I don't think 'termic' is a valid word. I've never heard it used. I looked it up in (the usually complete) Online Mirriam Webster dictionary - and 'termic' isn't there, either.
I then reviewed the Headway series ... I think I would call these 'graduated' courses or classes.
As you complete each 'class' or 'term' - you graduate to the next level/class/term.
Does this help?
Not a teacher, 53-year-old American.
Ö and thatís my opinion