I think I understand its intent. Families often send more than one child to the same school. School staff often track families by noting the grade of the oldest student in the family and the youngest.
Pōki'i usually refers to the youngest sibling in a family. Muli loa often refers to the youngest person overall.
If the prompt used muli loa, then the statement might be interpreted as "That student is the youngest child (of all the students in the school)."
So, my own interpretation is this:
He pōki'i kēlā haumāna.
That student is the youngest sibling (in his family).
I very much welcome other interpretations on this one.