It's the explanation for the red numbers used in my dictionary.
Who wants? Why isn't it just 身につける学年?
It expresses "generally expected". The usage is mostly similar to the English "you may want to include more contexts" as seen in comments on JSE. Technically the subject is you (which is the person who does the knowing) both in Japanese and in English, but of course the person who judges you want is the speaker.
It is the school year when "you" want to have acquired [the meaning of the entry].
There's no reason for using a pronoun. You could just say
the school year in which it is desirable to have acquired ....
I would hypothesize, since we don't necessarily know the mind of the people that wrote this, that 身につける学年 is too strong: the school year in which it is acquired. Perhaps for someone struggling to learn that would be too intimidating.
身につけたい is suggesting a target year; 身につける学年 seems far too confident about what you will achieve.