I live on the West Coast so I don't have any personal recommendations. I do know some people who teach at a studio in Boston called SuperShag. I would trust their recommendation for a teacher in your area, if you want to give them a call.
Otherwise, you can try the Directory on this site and at accessdance.com. Or google "ballroom dancing Amherst" and see what that turns up.
If you end up with a few possibilities, I'd recommend trying an introductory lesson with each to see who clicks best with your daughter. And that will give the opportunity to compare prices and studio atmosphere. Studios vary quite a bit in what they charge and how many lessons at a time they offer or require that you buy. Chain studios (such as Fred Astaire and Arthur Murray) often sell only packages that include private lessons, group lessons, and parties. They are only a good deal if you take full advantage of all of that. Since at your daughter's age, she might not want to take group lessons or go to parties with mostly adults, a chain studio might not be the best deal.
In my area, quite a few studios are starting to have group youth programs, so you might ask about that specifically.
I hope you find a great teacher and your daughter has a wonderful time dancing!
