At RSU#17 we have been doing what I would call a "hybrid" standards/proficiency based system at grades 7-12 for at least 5 years. It's a hybrid system because while we grade towards standards, we do still grade using percentages and averages. This means we do NOT use Power Law, which is only available for a "pure" standards based system (which is possible in IC - but which forces multi-level hierarchical aggregation issues).
In prior years we had a set of 2-6 "standards" per course (in quotes because in reality what we grade is way less specific than most standards... more like what you might call "domains" in the Common Core), and then teachers would do whatever they wanted in the gradebook as long as they covered each of these general areas... and of course, each standard averaged into a composite final grade so we'd still be able to have a regular GPA and class rank.
For core classes this year, we changed most of our standards to better reflect what we want to use for our Proficiency Based Diploma... so still roughly 2-6 very general standards - but more aligned to common core and NGSS etc..
Our other change this year was to force teachers to use Performance Indicators as their categories. We wanted the PI's to show in the portal - but without having too many standards in the gradebook (so as to not overwhlem our teachers) and this seemed like a balanced approach - but it is flawed... It took some doing in the database back-end, and still is pretty unwieldy (as PI changes get made, or new sections get added), but I'm almost managing to keep up... yet the biggest issue is that a per standard, an assignment may only be applied to one category. Whereas if our PI's were standards, then a single assignment could span multiple standards (useful in multi-score).
Long-Term we also plan to use the trick Will mentioned (and that I learned about at the last interchange) - so at the end of each course, the teacher will submit teacher-scored assessments per diploma standard, per student - as a way to manually sign off on whether each diploma standard covered in the course was (or was not) met. The tricky part here is that a single standard might be addressed in multiple courses (i.e. multiple pathways), but how then do we aggregate those on the diploma (mean, median, mode, power law, most recent, best,coin flip, etc...)?