We never know how high we are — Noisy Students

We never know how high we are 1
Till we are asked to rise
And then if we are true to plan
Our statures touch the skies-

The Heroism we recite 5
Would be a normal thing
Did not ourselves the Cubits warp
For fear to be a King-
(F.1197/J.1176)
[1] high:: exalted, hilarious.
[2] asked to rise:: a teacher asked the noisy students to rise.
[3] if, true to plan:: if students were true to their intention, they should rise and confess.
[4] statures touch the skies:: their great noble spirits, seemingly.
[5, 6] Heroism, recite, normal:: if they really did confess, then the heroic stories they read would be a normal thing.
[7] warp:: to become bent, twisted, or uneven, by shrinkage or contraction (OED 14a). Cubit:: the distance from the elbow to the extremity of the middle finger. King David's sling. ourselves the Cubits warp:: to shrink their body to height of cubits, not the skies.
[8] King:: a great person. King David.

And he took his staff in his hand, and chose him five smooth stones out of the brook, and put them in a shepherd's bag which he had, even in a scrip; and his sling was in his hand: and he drew near to the Philistine. ─ First Samuel 17:40