I think this may easily be related to Visa issue. Russia may have allocated this school as guarded shelter kind of things to avoid sleeping people of airport floor.

I have seen similar scenarios in case of irrops in Europe. We had LH fights BLR-FRA-SFO and BLR-SFO got delayed causing everyone to be stranded at FRA for about 20 hours. Anyone who had passport which allowed Visa free entry to Germany or Schengen visa were provided hotels and others (mostly Indian citizens) were literally provided blankets to sleep at the airport (they had a few portable beds setup in one corner at the terminal but not enough for everyone). Food was provided as boarding card credit (I think 20€ / 15€ per meal for J/Y)

I had Schengen visa so I went out but it must have been terrible for Indians stuck at the airport (especially seniors & children)

The same way, I guess Russian immigration won't clear the passengers to enter Russia so couldn't be sent to hotels)