A quick search on the website of NUI Galway turns up 92700 hits for mac léinn, 7640 hits for mic léinn, 406 hits for daltaí and 87 hits for dalta.
I think it's fair to say that mac léinn is the preferred term on the Irish University campus that has the closest association with the Irish language. The usage of mac léinn and mic léinn is also far more prevalent than dalta/daltaí at all the other Irish University websites that I checked.
Whatever the dictionary says, the normal usage in Ireland for 3rd level "students" is mic léinn. At both primary and secondary school level, I think you'll find a mix of daltaí and mic léinn in use, just as you'd find a mix of "pupils" and "students".
(Note that I'm not saying that dalta is directly equivalent to "pupil" and mac léinn to "student", just that those usages tend to parallel one another, somewhat).
Definition 2 at the FGB link above suggests that daltaí refers to the scoláirí scoile (school children, pupils) sense of “students“; definition 3 there suggests that it can refer to the mic léinn (university students) sense of “students”.
The Foclóir Beag entry for dalta offers no sense of scoláire, though it does offer mac léinn.
I can't explain the editorial decisions of different dictionaries (maybe that's why it's always good to use more than one source!) but as far as I'm aware if you ask someone what dalta/í means the first thing they'd generally say is a school student/s, school child/ren, pupil/s etc.
A college student could be insulted by being called a dalta, as it implies less knowledge, and mac léinn would always be used as more correct. You'd never call a young child a mac léinn.
This does not exclude that dalta/í has other meanings of course, it's just that if I heard or read 'Is dalta mé', on it's own, I'd assume they were a pupil, not a foster-child etc, unless the context was changed.
That might be the first thing that someone would generally say because the word was learned in an educational setting. If that person were referred to as a dalta in a classroom setting since the age of five (or whenever schooling begins in Ireland), then that could easily influence a person’s response to the question. (Note that in Dinneen’s dictionary, neither “schoolchild” nor “student” is given as a definition of dalta ; it seems to be a 20th century introduction. The eDIL entry for daltae confirms that that sense wasn’t present in Middle Irish or Old Irish; “pupil” was in the “disciple” sense, e.g. that of Timothy to Paul in the New Testament.)
Given that dalta has more than one meaning, I’d hope that a university student taking offense at being called a dalta had first considered the context in which it was said. Since a young child would never be called a mac léinn, the “student” definition in the FGB’s entry for dalta and the mac léinn definition in the Foclóir Beag’s entry for dalta should be taken into account by a young adult being called that.
The key point is that none of the exercises here have any context; they all stand independently, and so the meaning of dalta in this exercise could correspond to any of the word’s definitions.