Matter of Kane, 15 I&N Dec. 258 (BIA 1975) appears to have some relevance to this situation. Quoting from this decision,
Subjective intent can sometimes be determined from examination of such elements as:
(1) Purpose for departing. The traveler should normally have a definite reason for proceeding abroad temporarily. See U.S. ex rel Alther v. McCandless,
46 F.2d 288 (C.A. 3, 1931); examples are education and professional training,
Serpico v. Trudell, supra; employment for a definite, "albeit extended period," by accepting a two-year teaching position with a foreign university, Matter of Guiot, supra; ...
(2) Termination date. The visit abroad should be expected to terminate "within a period relatively short, fixed by some early event," U.S. ex rel. Lesto v. Day, 21 F.2d 307, 309 (C.A. 2, 1927); Matter of Castro, 14 I. & N. Dec. 492 (BIA 1973). If unforeseen circumstances cause an unavoidable delay in returning, the trip would retain its temporary character, so long as the alien continued to intend to return as soon as his original purpose was completed. For example, war might inhibit travel or the alien might be drafted,
Serpico v. Trudell, supra; there might be illness, Transatlantica Italiana v. Elting,
66 F.2d 542, 545 (C.A. 2, 1933); there might be a death requiring remaining abroad to settle
an estate, U.S. ex Polymeris v. Trudell, supra.
(3) Place of employment or actual home. The traveler must intend to return to the United States as a place of employment or business, Saxbe v. Bustos, supra; Gooch v. Clark, 433 F. 274 (C.A. 9, 1970), cert. den. 402 U.S. 995 (1971); Matter of Bailey, 11 I. & N. Dec.
466 (BIA 1965 and 1966); or as an actual home, U.S. ex rel. Lesto v. Day, supra;
Matter of D—C—, 3 I. & N. Dec. 519 (BIA 1949). He must possess the requisite intention to return at the time of departure, U.S. ex rel, Lesto v. Day, supra; and must maintain it
during the course of the visit, Gamero v. INS, supra; Matter of B—, 9 I. & N. Dec. 211, 213 ff (BIA 1961), reversed on other grounds, Barrese v. Ryan, 203 F.Supp. 880 (D. Conn. 1962).
In Matter of Quijencio, 15 I. & N. Dec. 95 (BIA 1974) we noted the significance of the location of the alien's ties, such as family, job or property, as an aid in determining the alien's intent. See also Santos v. INS, 421 F.2d 1303 (C.A. 9, 1970); Matter of Castro,
14 I. & N. Dec. 492, (BIA 1973), reversed on other grounds, Castro-Guerrero v. INS, 503 F.2d 964 (C.A. 5, 1974); Matter of Montero, supra; Matter of Salviejo, 13 I. & N. Dec: 557 (BI4 1970).
Based on these factors, it seems that a student at a four-year university abroad may be able to retain their LPR status on the basis of their trip abroad being for the purpose of education, having a termination date fixed by an early event (i.e., the student's admission to a school from which students typically graduate in approximately 4 years), and testimony that they intended to return to the United States upon completion of their education and retained that intention during the course of the visit abroad.
Under current immigration law, the student would need to return to the US at least once during each 12 month period in order for their green card to remain valid, or at least once during each 24 month period if they applied for and received a Reentry Permit prior to each departure.
If the student spent each summer in the United States with their family, this could be used as evidence that the student had, on each visit to the foreign country, intended to return to their home in the United States.
I have no legal training and no legal experience or professional experience of any kind relating to such a hypothetical situation and I am still hoping that someone with relevant experience could provide an answer. I am sure there are people who have been in this situation before and other people who are wondering about what would happen to them in a similar situation.