https://www.dur.ac.uk/od/mentoring/typesofmentor/inductionme...

http://uofg.esnuk.org/esn-uofg-buddy-system

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Note added at 10 mins (2020-02-28 22:44:23 GMT)
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https://www.bath.ac.uk/campaigns/campus-buddies/

https://www2.mmu.ac.uk/business-school/study/international-s...

some universities run the programme for "freshers", some for foreign students, and others for specific groups of students

If too much older and experienced veteran

My first thought is to juxtapose "junior" and "senior" to refer to new and older students at a school or college/university, with the latter (supposedly) having the knowledge and experience to help the former settle in and find their feet (practical, social and perhaps even academic support).

Despite no solid refs, and the specific meaning of "senior" in the US context to refer to a fourth-year student notwithstanding, the term sounds intuitive to me and I think it would be readily understood by any English-speaker.

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Off topic, I shudder every time I see "Anglo-Saxon" used to refer to anything other than the Germanic tribes that settled in Britain from C5. I know the term is widespread in French and (horror of all horrors!) is gaining ground in English too, but it should be avoided like the plague in careful writing. Use English-speaking, Anglophone or Anglosphere if that's what you mean or Anglo-American if you're being specific.

The Economist style guide puts it well:
"Anglo-Saxon is not a synonym for English-speaking. Neither the United States nor Australia is an Anglo-Saxon country; nor is Britain. Anglo-Saxon capitalism does not exist."

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Note added at 12 hrs (2020-02-29 11:21:14 GMT)
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See also https://forum.wordreference.com/threads/freshman-sophomore-s...

Does Ancien really mean étudiant plus âgé qui prend sous son aile un jeune étudiant pour l'accompagner dans diverses démarches au sein de l'école? That may be the case in context, which we don't have, but that is not what the term usually means.

And so the translation must reflect either the term asked or the definition given.

Ancien (esp. with a capital A as in the text of the question) stands for « Ancien élève », defined as “Qui n'a plus sa qualité antérieure (exprimé par le subst. qualifié). Ancien élève ” ( https://www.cnrtl.fr/definition/ancien), i.e. someone who is no longer a student in that school (and so, not just “étudiant plus âgé “).

The word "alumnus" comes to mind: “someone who has left a school, college, or university after finishing their studies there” (https://dictionary.cambridge.org/fr/dictionnaire/anglais/alu... or “ a person who has attended or has graduated from a particular school, college, or university “ (https://www.merriam-webster.com/dictionary/alumnus). See also Termium : http://www.btb.termiumplus.gc.ca/tpv2alpha/alpha-fra.html?la...

Note too that the American friends of A&M call themselves the “(816 Active) Alumni Network” (https://americanfriendsam.org/#g-contenttab-4).

We don’t know the destination country, but all English-speaking universities that I know use "alumnus" in that sense.

However, do Anciens really do that: “accompagner dans diverses démarches au sein de l'école”? That’s for bodies like the students’ association/union, or whatever they call themselves. If Anciens can do some of it (but within the school? ), what they mostly do is a lot of networking.

For that reason, I think that “peer mentor” fits at least part of the definition that we are given (“prendre sous son aile”) and could be the right idea here.