Two options. First, there is alumnus. For example, this text describes the life of Johannes Franciscus Du De Loz, in stages, from birth to death, including being an student of theology (presumably undergraduate), then a lector, professor, rector, and so on. There are several other examples in the text.
A second option is scholare gradus baccalaureatus, inspired from bottom paragraph here, and turned (maybe unsuccessfully) into genitive. Notice that scholares is in direct opposition to alumni (notice use of _sive, e.g. in page following that linked above). This expression would apply to any who is studying for a bachelor's degree, regardless of any previous status (e.g. graduate from another degree). This is, in my view more precise, as an undergraduate would be that who is pursuing "undergraduate" studies, instead of "graduate" studies. The latter is a common terminology nowadays, in my opinion.
(the second link provided are the regulations of the University of Oxford, from early 19th century. Maybe you can find a better expression there than I could.)