I'm sure I used to count things out, but there comes a point where playing things like duplets in 6/8 becomes a process of hearing as you read it on the page. I try to stay a bar or two ahead, and when I see something like that that, I can already feel my body and hear in my head the tug of the pulse. Maybe in a more crystal thinking age (i.e. when I was younger), I could count that out as I played it at tempo, but my sight reading's taken a dive over the years. I do more anticipating, and you're probably right -- in a tight situation, it will get me into trouble. Perhaps I've just grown lazy?
I understand, but just keep in mind we are giving advice to a
high school student. He needs to be able to count this out so he
understands it.

There's a lot of things we take for granted. We've done it for years so we no longer have to follow the same procedure. When I drive my car, I don't think about each maneuver like I did when I first started driving; much of it is second nature now.
So my point ... someone who doesn't understand the groupings of 2 in 6/8, or anything of the like, needs to learn to be able to count this out. Once you get it, you don't have to keep counting all the time. The point is that you count and figure out how it
should sound. Not doing this will allow the student to go through their school years not playing it correctly ... just guessing. When I was in school, I didn't know (and wasn't taught) how to play quarter-note triplets correctly. I just guessed, thinking "tri-pl-et" really slow in an attempt to get the three notes evenly spaced. That is ridiculous. There is an exact, precise manner in which to play quarter-note triplets; you can count them out ... it's just math.
That's where I'm coming from.

A young person or beginner needs to learn and understand the correct way to play/write this type of thing. The only way to do it is learn the theory and the math behind it. Count!