This is the poem, The Lesson by Roger McGough:

Chaos ruled OK in the classroom

as bravely the teacher walked in
the hooligans ignored him
his voice was lost in the din

"The theme for today is violence
and homework will be set
I'm going to teach you a lesson
one that you'll never forget"

He picked on a boy who was shouting
and throttled him then and there
then garrotted the girl behind him
(the one with grotty hair)

Then sword in hand he hacked his way
between the chattering rows"
First come, first severed" he declared
"fingers, feet or toes"

He threw the sword at a latecome
rit struck with deadly aim
then pulling out a shotgun
he continued with his game

The first blast cleared the backrow
(where those who skive hang out)
they collapsed like rubber dinghies
when the plug's pulled out

"Please may I leave the room sir?"
a trembling vandal enquired
"Of course you may" said teacher
put the gun to his temple and fired

The Head popped a head round the doorway
to see why a din was being made
nodded understandingly
then tossed in a grenade

And when the ammo was well spent
with blood on every chair
Silence shuffled forward
with its hands up in the air

The teacher surveyed the carnage
the dying and the dead
He waggled a finger severely
"Now let that be a lesson" he said

Well, after reading this well-written poem, you may feel that there is an irony in the poem. In the second stanza, the poet stated that the teacher who was frustrated and angry with his rowdy class wanted to teach them a lesson that his students would never forget. However, the teacher then took out his weapons and killed his students heartlessly. Most students were then dead in the carnage. As a result, no students would be able to remember this lesson by their teacher if they were already dead. Hence, here is the irony where the teacher was unsuccessful to achieve his objective.

Structure of poem (SLIMS):
In this poem, we can observe that it has 10 stanzas with 4 lines each. Each stanza has two lines that rhyme. Its sequence is couplet end rhyme, however, it does not have internal or sight rhyme.

After reading this poem, you may have some thoughts in your mind right now. You may be amazed by the way the poet describes how the teacher used his weapons and killed his students. I especially like the way the poet brings out the scene where students were shot by guns or killed by grenade. Hence, we can observe that the poet uses imagery in his poem to "put" the reader in the poem and helps the reader to "see" the scene in the poem. Imagery plays a vital role in a poem as it helps to create the tone and mood of the poem. It uses senses and figurative language to describe the scene in the poem. The figurative language used includes metaphors, similes, symbolism and allusion. One example of it is in the sixth stanza where "they collapsed like rubber dinghies".