Range has a slightly different meaning in the two cases.
The first case is about a range of values, from one limit/extreme to another.
The second, which I would express using "range among juniors, seniors, and graduates" is about an enumeration: any of the indicated values is a candidate.
In the second case, the enumeration could be understood as exhaustive (only juniors, seniors, and graduates) or it could be understood as indicative/exemplary (candidates include juniors, seniors, and graduates - and possibly others). Context should disambiguate, if only one meaning is intended.
Some might argue that "among" implies only. That's certainly one strong meaning, so "range among" has a fairly strong connotation of only these. But without further clarification it does leave open the possibility that there are other values that are also candidates.