Random Thought: wasn't the brainwashing thing going to wear off eventually ANYWAY?
I seem to recall from the story that Ms. Ririko had done these kinds of lessons with other students before Tsukune was ever one of hers and they eventually got better after the study session period was over and passed her course.
Even when she did it to Tsukune, he still subconsciously did things that were "normal" like approaching Moka whilst lacking in emotion (for a LIMITED TIME) and the point of these lessons were to instill him with the mathematics work he needed to pass HER course to go on summer vacation.
In retrospect, once he passed the class, he'd go back to normal with a decent amount of time to spend with Moka or Kurumu or anybody left, given the length of a Japanese schools summer vacation. It's not like he was going to spend it with Ms. Ririko with her continuing to clamp her tail on his head and input arithmetic into him.
If the logic is that she was trying to isolate Tsukune from Moka and his friends completely, she would have been trying to make him FAIL her final (because this was the FINALS of the school year) and make him repeat her class. Even if she went about it in a bizarre way, the end result was always going to be that Tsukune would pass her class and then spend summer with Moka and the others. I feel like there's a bit of a logic leap here.