What is the central idea of poem "The Village Schoolmaster"?

The central idea of the poem is that the villagers respect the schoolmaster for his education.

The poem describes a small village school.  Most of the residents in the village do not have much education, but the schoolmaster knows a lot by their standards.  They are very impressed with him.  Both students and adults admire his knowledge.

The schoolmaster is strict but fair.  He is not described as a tyrant, although he clearly does punish students who deserve it. 

Yet he was kind; or if severe in aught,
The love he bore to learning was in fault.
The village all declar'd how much he knew...

Although he is stern and the entire schoolhouse knows when he is having a bad day, he also seems to be a pleasant fellow.  He tells a lot of jokes.  The students laugh even when they don’t think the jokes are funny.  After all, this man is their teacher!

The villagers are very impressed that this man can read and write, because many of them can’t.  They admire the fact that one man can know so much.

Amazed the gazing rustics rang'd around;
And still they gaz'd and still the wonder grew,
That one small head could carry all he knew.
But past is all his fame.

What is never in question is the schoolmaster's love of learning.  He inspires his pupils to feel the same.  Even though they are "rustics," they have a chance to get a good education because he has one and is passing it on to them.

The poem ends with a sense of nostalgia.  The last line of the poem says that the schoolhouse has been forgotten, or the teacher has.  As time marches on, teachers are replaced and students leave to other things.  

Still, in his day, the schoolmaster reigned supreme.  He ran his schoolhouse with efficiency and dedication.  He doesn’t seem to have lorded his knowledge over anyone, but the villagers admired and respected it.  They may not have thought it was necessary to know everything, but they valued it all the same.

Of course today, this kind of education no longer exists.  It is rare to find a one-room schoolhouse. There is no longer one person that the village all reveres, because the rural populace became more educated.  This is a good thing, but we all still find romance in the idea of a tiny village with a one-room schoolhouse that everyone in the village learns from.