Astrid tells Sam and Quinn that Jink, and the other student Michael, have dyslexia. This is why they are in a class with Astrid, who is younger than they are. Astrid claims that they are both "math whizzes" and are better than her.
It is the disappearance of Jink and Michael, along with the adults, that first leads Astrid to make the connection between the poof and the age of those fifteen and over.
Jink is not mentioned for the rest of the series.