sophomore/ˈsɒfəmɔ/ (say 'sofuhmaw)noun 1. Chiefly US a student in the second year of a course at university, college, or school.–adjective 2. being or relating to the second in a series: sophomore album; sophomore novel.
I agree re sophomore, which while infrequently used in Australia, was certainly a surprise to me as uncommon.That said, as the complete antithesis of a mesomorph, when I chanced upon it, I did at least recognise it.
I suspect that it is not classed as common because it isn't used outside the US. Certainly, the rule "it's not common everywhere"...