Sounds just like my DD. So I'm in the same boat with wondering how it's going to work as she advances and needs split leaps and all... Here's our story so far:
DD started rec at age 6 3/4 1 hour/week just about 1 year ago. Her 'best' leg splits were still 9-10 inches away from being flat. Middle splits were probably also good 9-10 inches away after stretching. And her 'bad' leg - yikes. Super strong, not flexible.
After about 4 months of rec, and then a few months of pre-team (2x week), her middle splits have gotten to maybe 5-6 inches away from flat after stretching (obvious improvement), and her 'good leg' about 2 inches away (also obvious improvement). However, while other girls have continued to improve, hers have stayed there for the last few months with no improvement with just the 'in gym' stretching of twice a week, both at the beginning and end of class, including oversplits training.
Earlier this month we put a rewards chart up on our wall at home for DD and also my DS who is in the same boat. They get a "Star" for every day they do their warm-up+splits part of flexibility sequence at home, and a 'bonus' star if they do oversplits on a pillow/roller (all 3 ways). They usually do this in the 15-20 minutes before bed on non-gym days. If they fill up every day, they get to pick a special activity (movie, bowling, etc.).
In the 2+ weeks that we started this, I have seen more improvement than in the last 3-4 months of her only stretching in class. We'll see if this has longevity, but my DD now HAS HER SPLITS in 1 direction (good leg) after stretching, and is at least 2 inches closer on middles after stretching. She is so pleased with herself, which makes mom happy
I don't know what specifically your DD struggles with, but for us, after watching her for a year struggle with only the stretching in class not paying off enough, we decided to try the 'every day' thing based on some other posts here. I hope it keeps working for her sake! I think I am going to throw a big, freakin' party when she finally gets those darn middles!