I think that 20x200s is a better aerobic session for young HS girls than say 4x1000m. Both workouts are aerobic (for this level), but it would be easier on the girls by allowing them a break every 200m and giving them a chance to run consistently throughout the workout, also if a girl doesn't finish the session, they have atleast done something, even if it is 15x200m in 40s or something. If the girls are doing 1000m reps, they are going to be doing them in about 4:00 and only be able to do 2 or something. I think the 200s is a far safer option.
Also if you have a large group of different fitness it may be better as the ones at the back arent going to get as far behind (only a few seconds).... if they do 1k reps, then there might be 200m from the front to the back of the pack and it will just stuff up all their recoveries etc...
GO THE 200s!