Now it's time for Collins to take the idea of a poem having physical space even further.
Instead of a mouse in a maze, now we, the readers, are "walk[ing] inside the poem's room."
The word "room" is important here. Poems are broken up into stanzas. The word stanza is an Italian word that means room. The units, or stanzas, of a poem are like joined rooms in a house. It could be a 500-room mansion or a one-room shack. Either way, Collins wants us to take the grand tour and get a good feel for the place.
and feel the walls for a light switch
This line gives the sense of someone running their hand along the walls of a dark room looking for that elusive light switch.
Whooop, Whooop! You guessed it. Another metaphor. The light switch is a metaphor for understanding, inspiration, illumination, and the like.
It's as if Collins is telling us to look for the place in the stanza or in the poem that makes that light bulb come on.
Even in poems we think we don't understand, there are often moments that make sense to us, or moments we really like even though we have no idea what's going on. These are the light switches that the speaker is talking about—places that illuminate the darkness, make you think, make you smile, or turn you on.
The speaker (the teacher) is asking them (the students) to look for these places of illumination in the poems they read.