Nice to see people have forgotten phantom school, its probably my favourite place
The rooms aren't big but the spawns are super fast so it's a shame you don't see too many parties there.
I wouldn't say you need to be able to OHKO to do machine room though, my useless luck lions been ok there.. it makes the xp slower but I still liked it better than other places. For magic types once you can 1hit the keepers the 3rd floor toilet is like a little upgrade to the 2nd floor one, with keepers & inky muks it just gives a little more xp! Another room that I think used to be really popular for magics is the art room, they can hit hard but I don't think you need all that much MA to be able to OHKO. At 137 I still get decent xp there on my own so for someone 9-10x it should be really nice