The speaker is a teacher, or someone who sets exam questions, talking directly to a student. He admits that writing the questions gives him a kind of instinctive feel for them, one the student may lack. He pictures the student nervous and flushed, tongue-tied, struggling to keep a pile of ideas steady, like flimsy paper plates, while trying to push through the "carefully arranged curtains" of the question. The last line of that stanza is self-mocking: the student's task is to discover what "a clever old fool" the examiner is.
The mood shifts in the short, stark line "At worst's worst." The speaker imagines the student shattering like an ancient ceiling, while he himself ends up buried under "a thousand crumbs of artfulness," smothered by his own cleverness. The tone mixes sympathy with guilt and dry humour. He seems to suspect that exam trickery can hurt the person it tests, and perhaps empty the person who sets it.