As a teacher, I can imagine that beneath the surface of Roger McGough's poem, "The Lesson" is a sense of frustration. It is this that I believe is the theme of the poem: frustration of teachers over the ineffective actions a teacher may take to bring order to the unruly classroom. In a certain environment, a polite request will do. However, there are many times, as seen in this poem, where the teacher feels that options available to him or her are inadequate.
Because the choices portrayed in this poem are so far from acceptable, it would seem that this poem is a satire, which uses exaggeration to make fun of a person, idea, institution, etc. Satire is harsh (whereas "parody" is generally more playful). What little humor there is in this poem (that comes from wordplay) is extremely caustic.
Generally, a theme is a life-truth that an author is trying to get across to the reader. Symbols are items that appear within a piece of literature that are not generally to be taken literally, but represent something else. (E.g., a dove represents "peace.") I see the theme to be (even though the teacher says it's "violence") that the teacher and/or educational system has lost the ability to control its students: often the students seem to control the classroom, and the teacher is beyond frustrated. The symbols (which often support a theme) are found in the form of weapons. The exaggeration of the satire (to me) is that nothing short of all-out violence will force the recalcitrant students to cooperate.
The title of "The Lesson" is a double-entendre. The teacher is presenting a lesson, but by punishing (killing) the students, he is teaching them "a lesson / one that you will never forget," as is noted in the second stanza of the poem. It is echoed again at the close of the poem:
"Now let that be a lesson" he said
The two associations with the word "lessons" are very different: one is referring to a classroom lesson; the other refers to a life-lesson... often times shown in old movies or old cartoons when a child was receiving a "spanking."
In terms of the symbols, we are presented with "tools of destruction," such as bare hands, a garrote, sword, shotgun, and a grenade (tossed in by perhaps a member of the administration, nodding "understandingly").
Those who the instructor is attempting to teach persist in talking loudly and ignoring the teacher; shouting, chattering, sciving, coming late to class, and destroying school property (vandal).
However, there is one piece of very dark humor—seen with the play on words of the cliche "first come, first served:"
Then sword in hand he hacked his way
between the chattering rows
"First come, first severed" he declared
"fingers, feet or toes"
In this case, the first to come is the first to have "finger, feet or toes" severed (or cut off).
The symbols appear as weapons, but perhaps they represent things that will have impact on students: making them clean a classroom or cafeteria after school; making them take part in community service projects on weekends; taking cellphones or laptops away—things that make students take notice, when a phone call or note home is meaningless, a detention is cut, or a suspension feels like time off—a reward rather than a punishment.
Success is measured by...
Silence shuffled forward
with its hands up in the air
The poem may also be the poet's way of suggesting that something must be done to take the control of the classroom back. His message is not to be taken literally.