In the US, we're pretty specific in using juniors (11) and seniors (12). So you can't just say seniors because people will think you're talking about Grade 12 students. I don't know of any special term for these two grades referred to together. Maybe upperclass students. Maybe.How do you call senior high school students?
I mean those grade 11-12 students.
Since we're speaking of AE here, I don't see the problem.In BE, if you were to use the term 'upperclass', you would risk it being read as 'upper class'. A book claiming to be just for upper class students would cause a riot!