Being around for her Senior Year doesn't automatically mean that Steve is referring to next year. The implication works in that, Steve would be around for the remainder of Nancy's High School years and onward, if he doesn't get into college.
World Turned Upside Down has a Foreward by the Duffer Brothers, and is subtitled as the Official Behind the Scenes Companion. With how meticulous the production is to detail, I think getting Nancy's High School Year incorrect out of everyone elses, is highly unlikely. In WTUD the 1984 students are listed as: Steve (Senior), Jonathan (Junior), Nancy (Sophomore) and Barb (Sophomore), even though she is dead.
I thought the same thing as you at first, but it lines up that Nancy would be a Junior, and Jonathan a Senior or possibly a drop out by ST4. In ST3 Jonathan was more concerned then Nancy about losing the internship, since he was closer to graduating, so it would go to reason that Nancy was less so, because she had two more years to improve her college application.