Is that a real thing? I know Norway is a tiny country but are there 7-student classes?
Oh yes. Not in the cities, of course, but in sparsely populated areas.
Also, my uni class has seven students.
In the remote areas perhaps but at the University level? And NOT only because it's a very niche course? Mind == blown. Can you provide any concrete source on that? Any English-speaking one I've looked into so far has been unhelpful or anecdotal. Even OECD (https://stats.oecd.org/Index.aspx?DataSetCode=EDU_CLASS)
7 students in one class at university level usually means the course is either really niche, or other restrictions are in place. (Ie. I know the best "universities" that teach singers in norway only take 2-3 students a year.) However, whilst most norwegians will tell you this happens, they can usually not name more than one in such a study at university level. (Basically, it happens, yet one should not come to norway expecting there to only be 7 students in a class as it is considered highly unusual.)
Deliciae mentioned that sparsely populated areas might have classes where only 7 students attend a class. If they are talking about primary school, that is a known fact. However, that their class in university only had 7 students was listed separately, making it a singular example of an university with a class where only 7 students attended.
Depending on the degree and school, my guess (here from personal experience) is that 1st year on university tend to have all from 50-300 students attending the general courses.
Now, you asked for sources, yet I feel like you are mainly looking for statistics that show that an average university has only 7 students per teacher(?). There is none. If we are talking about general subjects (like someone studying to become a teacher, etc) then it is really really uncommon and case for concern.
However, if you want to know if schools (pre university level) have that few students, I'll happily to dig up some statistics from the more sparsely populated areas. (Though it might border into naming x amounts of schools, given the government isn't exactly mass-producing schools where the population is sparse.)
Unless few are interested in the course as a general, schools are only made for few people in the cases where the students are too young to live apart from their parents.
Now: Looking at the main topic, which is learning the norwegian language, "læreren har bare syv/sju elever", is a sentence you mostly need to know when you speak to someone that is from one of the sparsely populated areas. Basically, it is a phrase used to illustrate how few actually lived there. (Here the inclusion of "bare" (=only) is used to amplify that the following number is considered really low.)
And whilst it might seem odd to have such a specific sentence in a language course, it is mainly there as a sentence constructed out of words already learned. The purpose being to teach the use of the words, rather than a fact about norway.
Funny, at my old school we were thirteen and at my new we're eight students only. :D