Man, this is going to be one tough question to answer truthfully. It's not like we all get to study dentistry at more than one school.
I would like to say yes, some schools are more challenging since every school has different profs, and different profs have different requirements and exam making habits. However, I would also like to say no, all schools are the same overall difficulty since the ADA requires that all schools have a cirriculum that prepares all students to be competent both academically and clinically (NBDE and Regional Board Exams).
As far as curving goes, Tufts grades on a curve in some classes while other courses are not curved at all. I.E. gross anatomy was curved, while biochemistry, dental anatomy/occlusion, nutrition, and ethics were not. I've only completed one semester, so I can't speak for any other courses, yet.