What if there is exactly one student with 6 achievements, and other students have 0? This makes the gamestate illegal.

And i guess it can’t work altogether:
“When a Rule causes a Student to Score, that Student gains an Achievement. <...>
This Rule takes precedence over other dynastic rules for the purpose of determining when or whether a Student Scores.”

Re. the first point - the student can’t get to 6 achievements due to the last sentence. Re. the second point - as this is a subrule to the rule cited, it is part of that rule and thus both slips inside and also adopts that restriction.

Ah, got it. 

[Josh] They could get to 6 Achievements while it was legal, and other players could then lose some Achievements (perhaps only by idling) to make it illegal.

Still not really sure of the best way to approach this kind of thing; the player-enforced “if X>10, any player may set X to 10” is, I think, the easiest way to stop the gamestate getting tangled, but we have to live with the possibility of obscure, stretchy exploits.

This appears to be ineffective because Rule 2.2 says it takes precedence.

Bucky, that’s addressed in the comments above.

What if Kevan and I both have 6 achievements, but no one else has any, then then I idle?

Yes, that definitely needs patching.

I like it, but I prefer catch-up rules to slow-down ones. Plus the idling thing. 

Subrules are considered part of the parent rule now?

Subrule
A subrule is a type of rule that is nested within another rule.

Okay, fair enough.  , though, because the number of Dynastic rules is a more natural limit.

I prefer a proposal that redistributes non-scoring Achievements to those with the fewest Achievements