kono koutei kara miru aki no kouyou hodo kireina kesiki ha mettani arimasen.

A useful way to chunk it is:

• この校庭から見る秋の紅葉ほどきれいな景色 = scenery as beautiful as the autumn leaves seen from this schoolyard
• は = topic marker
• めったにありません = is rarely found / hardly ever exists

So the whole sentence means something like:

• Scenery as beautiful as the autumn foliage seen from this schoolyard is rare.
• Or more naturally: You rarely find scenery as beautiful as the autumn leaves seen from this schoolyard.

The long part before は is one big noun phrase.

Here, ほど marks the standard of comparison and means to the extent of / as ... as.

Pattern:

• A ほど + adjective + noun
• a noun that is as adjective as A

So:

• この校庭から見る秋の紅葉ほどきれいな景色
• literally: scenery that is beautiful to the extent of the autumn leaves seen from this schoolyard

In smoother English:

• scenery as beautiful as the autumn leaves seen from this schoolyard

Also, this is not the ほど meaning approximately. It is the comparison ほど.

Because この校庭から見る is modifying 秋の紅葉.

Japanese uses clauses before nouns, where English often uses relative clauses like that you see or seen from.

So:

• この校庭から見る秋の紅葉
• literally: the autumn leaves [that one sees] from this schoolyard

Natural English:

• the autumn leaves seen from this schoolyard
• the autumn foliage as seen from this schoolyard

This is a very common Japanese pattern:

• [verb/clause] + noun

For example:

• 昨日見た映画 = the movie I saw yesterday
• 駅で買った本 = the book I bought at the station

It is omitted.

In Japanese, subjects are often left unstated when they are obvious or general. Here, the idea is something like:

• (you / we / one) see from this schoolyard

So この校庭から見る秋の紅葉 does not mean the schoolyard is doing the seeing. It means:

• the autumn leaves as seen from this schoolyard
• the autumn leaves you can see from this schoolyard

The omitted subject is just a general viewer.

Here から means from, indicating the viewing point.

• この校庭から = from this schoolyard
• この校庭から見る = to see from this schoolyard

So it tells you the location or vantage point from which the autumn leaves are viewed.

Because きれい is a na-adjective.

When a na-adjective directly modifies a noun, it takes な:

• きれいな景色 = a beautiful scene / beautiful scenery

Compare:

• 静かな町 = a quiet town
• 有名な人 = a famous person

Even though きれい ends in い, it is not an i-adjective.

Because the sentence is presenting 景色 as the topic:

• この校庭から見る秋の紅葉ほどきれいな景色は
• As for scenery as beautiful as the autumn leaves seen from this schoolyard...

Then the sentence comments on that topic:

• めったにありません
• there rarely is any

This kind of X は ありません structure is very common.

It can sound a little more formal or literary than a simpler everyday wording.

めったに means rarely / seldom / hardly ever, and it is usually used with a negative verb.

So:

• めったにありません = there rarely is / there is hardly ever
• めったにない = casual form

Examples:

• そのような機会はめったにありません。 = Opportunities like that are rare.
• 彼はめったに来ません。 = He rarely comes.

So in this sentence, the idea is:

• Scenery that beautiful is rarely found.

Because 景色 is an inanimate thing.

Japanese uses:

• ある / ありません for things, places, abstract things, events
• いる / いません for people and animals

So:

• 景色があります = there is scenery / a view exists
• 犬がいます = there is a dog

Since 景色 is scenery, ありません is correct.

In this sentence, 紅葉 is most naturally read こうよう.

So:

• 秋の紅葉 = あきのこうよう

Why not もみじ?

• もみじ often refers to maple leaves themselves, or can be used as a more concrete noun.
• こうよう often refers to autumn foliage, the changing colors of leaves in general.

In a sentence like this, talking about a scenic view, こうよう is the most likely reading.

It can feel a bit unusual if translated too literally, because English may separate leaves from scenery more sharply.

But in Japanese, this is natural. The idea is that the autumn leaves viewed from this schoolyard form a beautiful scene/view, and that scene is being used as the standard of beauty.

So although the wording is literally close to:

• There is rarely scenery as beautiful as the autumn leaves seen from this schoolyard

the intended sense is more like:

• Few sights are as beautiful as the autumn foliage seen from this schoolyard.
• You rarely see a view as beautiful as the autumn leaves from this schoolyard.

So the comparison is really about beauty as a visual scene, not just about individual leaves.

Yes. Depending on style, you could translate it as:

• You rarely find scenery as beautiful as the autumn foliage seen from this schoolyard.
• Few sights are as beautiful as the autumn leaves viewed from this schoolyard.
• Scenery as beautiful as the autumn foliage from this schoolyard is hard to find.
• There are hardly any views as beautiful as the autumn leaves seen from this schoolyard.

All of these reflect the same grammar, just with slightly different English phrasing.