It may be bugged for you I don't know. After refusing her the final time the next day you automatically have her come after school for a private punishment. I would say don't automatically skip through days if you were and maybe don't have any other girls come after school for punishments and maybe that will trigger it for you? On my playthrough it would have fallen on a friday but cuz of the board meeting it didn't happen til monday. Also you won't get the option to summon her after school until after you've completed this and assigned her the role of test subject. So if you have the option to summon her but are still getting the chastity panties scenes it sounds like a definite bug. In that case hopefully you have a save game point from before this path that you can revert to and try again without having to play from beginning?