Would it be proper to say freshman students, freshmen, or freshmen students?
It is worth noting that I have since learned it is more acceptable in educational circles to use the term "first-year students" instead of "freshmen".
Both "freshmen" and "freshman students" are correct, but "freshmen students" is wrong. I think "freshmen" is the official term.
In Canada, or at least in the part of Canada where I went to school, you can also say frosh. The singular and plural are the same. It can also be used as an adjective, as in frosh week, the first week of the term when the frosh are introduced to the school (and local pubs).
Looking at the definition of freshman on the New Oxford American English, I find two examples:
We invited the freshmen.
[as adjective] A freshman second baseman.
In English, when a word is used as an adjective, the plural form of that word is not used.
(*) We invited the freshmen students.
You can also say "freshers", although I'm not sure if it's proper.