I'll throw out my two cents and offer this simple advice. Check her feet. If she has too long of toe it really does seem to throw a pace. If her feet are good, then just take her down to the Walk and correct her when she paces.
By going back to the walk, and spending time developing that 4 beat walk, it really helps to break up that pace. I found that especially with a snaffle (is
..that the bit the teenager worked her in?) they can gait but it takes a little bit more effort on your end especially if they are used to using the bit as leverage to gait.
Just walk, squeeze with your legs and have a firm by yielding hand, and get a more forward walk. As soon as you hit that stride give the "good girl" and as soon as she starts getting pacy, take her down to a flat walk, then cue for the RW.
Here are my "cues" for gaits, and everybody is different but this is what works for my multigaited Standardbred:
Forward Walk (Running Walk on my TWH) is a light hold on the mouth (as you would for cuing a trot) and squeeze with the thighs.
Rack: I hold my hands up higher (my walk is lower almost on withers same as trot) sit a little deeper on my hip bones, and squeeze and give a "kissy" noise.
Canter: If going to the right, tilt head slightly to the right to open up the left shoulder, sit on my hip bones, and with my right leg behind the girth along with "clucky" noise cue for canter. Opposite if going to the left.
My OTSTB also does a Foxtrot but I've yet to get a cue for that as he is really inconsistent with even picking it up and he seems to slide into it when I ride with Foxtrotters. He will almost Running Walk with TWHs and his rack is just smooth as silk. His canter is actually really good, he just doesn't do it well under saddle, yet picks it up wonderfully when doing marathon in CDEs.
Good luck!! My TWH mare is very pacy but she is in need of a lot of groceries and her feet is a work in process. She hasn't had proper farrier care in months and her feet are really bad. Her gaits at liberty are just breathtaking and with those bad feet, so hopefully she can keep those with healthy feet :)