The next day at lunch, stupid me, I sat down at a table with Tristan, Nino, and Pablo. I thought maybe they were safe because they weren't really considered popular, but they weren't out there playing D&D at recess, either. They were sort of in-betweeners. And, at first, I thought I scored because they were basically too nice to not acknowledge my presence when I walked over to the table. They all said "Hey," though I could tell they looked at each other. But then the same thing happened that happened yesterday: our lunch table was called, they got their food, and then headed toward a new table on the other side of the cafeteria.

What does "in-betweener" mean here?

Question

The next day at lunch, stupid me, I sat down at a table with Tristan, Nino, and Pablo. I thought maybe they were safe because they weren't really considered popular, but they weren't out there playing D&D at recess, either. They were sort of in-betweeners. And, at first, I thought I scored because they were basically too nice to not acknowledge my presence when I walked over to the table. They all said "Hey," though I could tell they looked at each other. But then the same thing happened that happened yesterday: our lunch table was called, they got their food, and then headed toward a new table on the other side of the cafeteria.

What does "in-betweener" mean here?

Answer

In-betweeners

This is basically the same as "on the fringe of popularity"

The writer is again making a point of the kids he/she hangs out with as being average.

Or not unpopular but also not popular enough.

Answer

It is a slang and refers to A teenage Sub-Group, ie. People who are not cool enough to be popular but are not nerdy enough to be geeks.
Inbetweeners are therefore an average of the two major teenage social groups: geeks and popular ones