when playing as Nemu it is not possible to start class, and I believe I know what causes it: script/story/nemu/advanced classes/1. only izumi.rpy:1476
it reads
$ game.world.introduce_object(key = "start class a")
but this is Ayumi's classroom (and is unlocked on his path correctly)
I think this should be
$ game.world.introduce_object(key = "start class b")
(and you have to think about a way how to retroactively fix it, can be probably solved within the special label "after_load" and something fugly around renpy.seen_label(advanced_classes_only_izumi_end_label))