"There are a thousand students in total."
Translation:Hay mil estudiantes en total.
I googled "un mil" and several sources said "Just don't - it's wrong"
I think that is just what we do in English. We only use "a thousand" and "a million" and "a hundred" ... but we never say "a one, a two, a twenty" ... so I think in Spanish, none of their numbers are preceded by "a" , at least not a thousand.