By definition, any non-Arcavian attending Strixhaven must be a planeswalker*. According to Magic lore, planeswalking is the only way for living creatures to travel between planes, and they can't take passengers except in a few, very specific cases (e.g., Yanggu/Mowu). This was a major plot point during the Bolas arc, where Bolas had to go to great lengths to craft an undead army that could be transported between planes.I see. I looked them up and all three of them are Planeswalkers though, spark and all.
Presumably, the vast majority of Strixhaven students are Arcavian, but one would also expect it to attract a fair number of magic-using planeswalkers seeking to hone their skills. In D&D, of course, planar travel is much easier, so it's likely the contingent of extraplanar students would be larger than in the official M:tG continuity.
*Or else really, really old--predating the Mending.