I'm a 22 yr old girl in England (where it is very grey by the way and we'd like some sun from the states! - guessing you're in America cos unless you went private in the UK you'd never get a routine MRI that quick!).
I had a similar accident about 3 years ago and thought it was just a bad spain and left it , though couldn't weight bare fully for 3 days. I had very recurrent re-injury of the knee like a lesser version of the original injury (about once or twice a fortnight it would re-sprain).
I have been having it looked at by various very experienced physio's over the past 6 months while I've been on a waiting list to see an orthopaedic consultant, and the original thought was a classic ACL rupture and meniscus tear, as the laxity tests were positive.
However, I saw the ortho consultant and after much pushing and pulling (he really pushed my patellar across laterally at which point everyone went "ah..") it looks more like I've been getting patella subluxation/dislocation, and the laxity felt in my knee before was due to a lot of hypermobility (hyperlaxity - runs in families, benign condition).
It really confuses diagnosis and they want to check out my ligaments too so now i'm in the MRI queue! (waiting list here for routine MRI where I live is 2 months but that's pretty good really - I thought it would be more like 6 months).
Anyway, as a total guess i reckon if the ACL was completely torn they'd see it on the MRI even if it isn't so clear in teenagers, as it's usually pretty obvious (as far as i'm aware!) If it was her patella clicking out of place the main thing they do is loads of physio to get the muscles strong and make it run back on track. The physio can also advise as to whether she should wear a knee support for sport, and what type to get.
Hope that helps or something, feel free to get in touch if you want!