Can a single Morah and a Bachur teach on the same floor that no one else is on?
If this is a public building, the Bachur may be on the same floor, since there are others who often come to that floor for pickup, drop-off, or other things.
Of course, they should ensure to have proper fences in place to prevent unnecessary interaction.
In a private home based playgroup, or home school, however, Bachurim shouldn’t be teaching. There are also Halachic issues with women, married or single, running a private playgroup from home.
See here for possible solutions.