From the NY times:
"Temperatures had become stifling, survivors and their families said Saturday . . . . .
"On the first day, they were given buckets of water and cups, but not enough, and people grew parched with thirst. The captors took clothes, soaked them in water and threw them to the crowd, who clutched them and wrung them above their open mouths, drinking the drops . . . .
"People did whatever they could to take care of themselves, shedding clothes to cool down, and tearing apart school textbooks to use as fans. "For two days I was continually waving my arm to fan my children," Ms. Bekoyeva said. "They kept asking for more."
"By late Friday morning, as the temperature soared anew in the crowded gymnasium, the hostages were becoming weaker by the hour. "People were almost losing consciousness," Azamat said."
The adults and children were in a state where normal inhibitions no longer apply.
Elsewhere I have read that children urinated into bottles and drank it.
I have no doubt they also stripped to their underwear.
As for any who may have emerged naked from the school--it is just as likely their undergarments were blown away from the force of the explosion and/or consumed by fire.
So kids in their underwear or even naked is not proof of itself they were assaulted.
Maybe not proof --- but I think they were assaulted or at least sexually humiliated. We'll see what comes out in as investigations are done.