A few "off-the-top-of-my-head" completely unofficail thoughts...
Every time I've done a flight with a touch-and-go at an airfield other than where I've set off from, I've logged it as one flight (it's only ever been in the USA, as I always have to stop to pay landing fees over here!). That includes my CPL skills test, when I did the circuits away from my home base, and the CAA didn't query it.
I don't see any reason why an instructional flight should be any different. However, it can't count as instruction until after the touch and go, so the instructional time must be less than the total time of the flight. So, as long as your logbook allows you to keep track of the instrucitonal time separate to the total time of the flight, I can't see why there'd be any problem logging it on just one line.
As I said, this is just my thoughts, please don't take it as official in any way!
What I would be curious to know is how your students log the time.....
FFF
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