There is a grave error with the plotted murder here, and it involves dragging this act out so far from home only to bring it all the way back to the school to make it look like an accident. In Les diaboliques, Christina Delassalle (Véra Clouzot) plots to kill her husband Michel (Paul Meurisse) for his abusive and cheating ways; however, she plots this execution along with one of the mistresses, Nicole (Simone Signoret), because she too, is sick of him. So they go all the way out some ten hours from where they live to Nicole’s vacation house, bait Michel to go all the way out there, then drug him with a sedative in his whiskey and drown him in the tub. Then they plan to drag his body all the way back to the school, dump him in the pool, and wait for someone to find him, leading to believe that he “accidentally” drowned.
Realistically, this is an insane amount of work. What she should have done was start off the same. She said she was getting a divorce so this would have angered him to come out and try and bring her back. When he got there, she should still have offered him the whiskey with the sedative to get him to fall asleep. A key moment was definitely to have the whiskey still be spilled on him, so he'd want to take off his clothes and take a bath. So, after Michel passes out from drinking everything, she should have stripped off his clothes and left them crumpled on the floor so it would look like he was very drunk (and smelled very drunk) in this process. Then, drag his body to the full bath where people would believe he could have drowned in an accident. At some point shortly after she was sure that he was dead, she should have left for a walk to have witnesses believe she went out for a stroll because Michel was angry and wanted to be left alone. When she got back she would then call the police and claim that her husband fell asleep in the tub, drunk, and drowned. Maybe even have the bottle upturned and empty in the tub as well. Therefore, she could claim complete, believable innocence.
But, as we know based on the end of the movie, none of that really matters.