Of course, in a fight she can probably just load her power into her limbs and be freaky kinds of tough.
Hmm, that's probably more Ranma oriented than SM.
Back on topic...
I know it's not quite canon, but I think I'm going to got with Hotaru being in either 7th or 8th grade, since it's after all of SM, which, theoretically, could have taken a year after Hotaru's entrance.
Back to slightly OT,
It depends if I keep Chibiusa in the plot or not. (Usa would be a year younger, school wise.) It's tempting not to, since having her in could spoil everything (a la The Best Of Times), unless I just use a variation of her not knowing, since...sigh....cliché alert....
(monotone voice) Ranma's chaotic life interferes with the Time Gates, so Usa didn't know about it somehow, since Ranma's chaotic life messes with the future, so Pluto couldn't see it...
Unless the gates were still somewhat closed, as mentioned in the manga.
Later,
MV