hi, bejewled.
mmm; getting a schoolhorse who may very well may be dead to your leg to listen to you can be a tremendous pain, eh? there are lots of things you can work on to get a horse to move off your leg more effectively, but of course, it's hard to work on these things when you're in the middle of a group lesson with 6 other horses and riders running around, not to mention the fact that you've got an instructor who may not be paying too much individual attention to you.
if you'd like to think of this as sort of a "project", ask your instructor if you can have a private lesson or three on this mare, if that's convenient and affordable enough for you to do. it'll be a lot easier to work on this without all those other people and horses getting in your way!
before you start working on things like bend in corners and circles, try getting her to listen to your *lighter* leg aids. here's how i learned it: schoolhorses are often kicked away at so much that they learn only to respond to huge amounts of pressure (an 8 or 9 on a scale of 1 to 10). if possible, it'd be good for the mare to respond to a 2 or a 3, so you can get more out of her when you need it, like, say, in an extended trot or any upward transition. perhaps your instructor will allow you to work with a crop - say you want to go from a walk to a nice working trot. give what you consider to be "normal" leg pressure. if she responds, FABULOUS, give her lots of praise. if she doesn't (and she probably won't), TELL her want you want instead of only asking for it - give her a sharp tap with the crop or a quick kick. the important thing to remember is to not go on giving her more and more leg pressure; she knows she can get out of doing work by not listening to you until it's absolutely necessary. so, when she doesn't respond to a 2 or a 3 on the scale of leg, give her a 9 or a 10. when she moves in response to that, praise her like mad. even if she responds with a little more trot (or even canter) than you'd like, let her go for a few strides, and then ask her for a walk. by the time you do this a few more times, she may very well figure it out! then you just *might* be able to use your inside leg to get some bend out of her when you need it.
so, to make a long story short - try to work with the mare individually for a while, then go back into group lessons. by that time, you'll ideally have a better relationship with her, and she won't try to get away with bad behavior like running all over the place.
hope this helps somewhat (and it isn't too detailed) - good luck!