You can create multiple entries in the schedule for training, like this:
(https://i.imgur.com/OKAQggU.png)
This is useful to encourage some marksdwarfs in the squad to use archery targets, and the others will train together (sparring, demonstrations, etc).
To encourage sparring, I recall setting several "Train, 3 minimum", but I last did this in 44.nn.
A Sparring Demonstration might have different requirements than Sparring, are the other dwarfs in the squad doing "lead/watch sparring demonstration"? If yes, does the leading dwarf have skills?
No, they're actually sparring, as seen in combat logs:
(https://i.postimg.cc/Wb0qFXCy/sparring.png)
This is one of the recruits in question:
(https://i.postimg.cc/9X6qQv8T/recruit.png)
Most of the citizen militia members have skill levels similar to Tulon here. Or used to have, a game year has passed meanwhile and they have progressed quite a bit through training and sporadic sparring.