In my (albeit limited) experience, your dwarves will only spar if they are of a similar skill level to their potential sparring partners. This holds true in larger groups from what I've seen.
So if you have a Level 5 axedwarf/fighter, and a bunch of no-skill recruits, he'll spend his time Lecturing and Demonstrating his abilities rather than sparring till the others are closer to his level. That's not to say the Recruits might not get up and spar with each other, but he's likely to draw their attention for group lessons. Makes sense if you think about it.
This can be both a good and a bad thing. Your leader dwarf may decide to give a demonstration in Biting or Kicking if he has skills in it, so while (arguably) useful, it's more useful if they're training to exclusion on a particular skill - this is where sparring tends to shine. However if handled right and the skills align dwarf whom is ONLY skilled in 1 or two big skills (IE: Axe/Shield) will tend to only give lessons/demonstrations in those. These can be quite effective for a large group especially if he has a high Teaching skill and the Students with their respective Student and Concentration skills tend to also get bonuses.
I dislike the 2v2 sparring nonsense that's spouted. Makes for decent fighters, but tends to eschew the Esperit de Corp (Or however it's spelled...) and I like to think they're a more effective team when they fight/train together.
I don't think it's a thing though I'd love to see it, but having the dwarves tend to assist each other in combat more often and sticking closer together would be an advantageous benefit for group lessons over the current combat system.
Anywho, that's my 2-cents. I'm sure someone will toss in a nickel and Dwarf (pun intended!) my thoughts/approach. There may be a mix of good/bad info here as I myself am still learning, but I think most of it holds up and stands as true - or close enough as to not matter.