I'm looking for a word for what you'd in the workplace call 'seniors', i.e. "My seniors told me where the cafeteria was in Building 1." But in college, 'senior' has a different meaning, and "upperclassmen" doesn't quite work. How should I refer to students in the years above me?
Here in the US, we do not use seniors to mean those above you in the workplace. Generally, if they are higher than you in the organization, they are called superiors, although if ranking by time served grants you certain privileges (like who gets laid off last), you may say you have seniority over (or even you are senior to) someone else. If they are below you, they might be called your underlings, or occasionally, inferiors. If they are merely older or younger, or working there longer/less time, there is no specific term for that relationship.
In US high schools and colleges/universities, junior and senior have specific meanings; respectively, third- and fourth-year students. First- and second-year students are, respectively, freshmen (which is traditionally gender-neutral, but problematic to some, because of the "man" in it) and sophomores.
So that leaves the problem of what to use to refer to students in grades above or below yours. The only solution I can come up to make clear what year you are and then refer to the specific year students you mean:
As a sophomore, I find the insights provided by the juniors and seniors to be very helpful in making my course selections.
Of course, if you are a freshman, using upperclassmen could mean "everyone else, from sophomores to seniors." (Many dictionaries say that an upperclassman is either a junior or senior.)
In college we don't typically refer to others as senior or junior (in the US at least). The closest we have is upper and lower classmen, but they aren't used often. I'd avoid it, unless you are trying to point out their advanced knowledge (use "Senior", "Junior" or "Sophomore").
I'm not sure if this answers your question, but it might help: https://ell.stackexchange.com/questions/60815/how-could-i-address-students-in-a-higher-grade-than-me