Below is a link to nice overview that explains that Maine’s LD 1422 (passed in 2012) mandates a “proficiency diploma” for graduation — it does NOT mandate Proficiency-Based Learning/Proficiency-Based Education as the only means for districts to meet this requirement. You can view Maine’s statute here.
A key excerpt from the post (linked below):
This method [proficiency-based learning/education] is not part of the law, nor is it a recognized system validated through rigorous and objective research. Rather it is an amalgam of tools and methods that the department has embraced and advertised as necessary to put the law in place. It [Maine’s Department of Education] mounted an entire website devoted to it and sent out a slew of consultants to help schools adopt it. Bearing the education department seal of approval, this method has permeated almost every school and classroom in the state without legislative mandate or public discussion and debate. It has erroneously become viewed as having the weight of law.