kono tatemono ha katute koukou datta.

It is read:

この たてもの は かつて こうこう だった。

Word readings:

• 建物 = たてもの
• 高校 = こうこう
• だった = plain past form of だ

この means this and must be followed by a noun, so この建物 means this building.

By contrast:

• これ = this one / this thing
• この建物 = this building

So you cannot say これ建物.

は marks the topic of the sentence. The topic is この建物 — as for this building.

So the structure is roughly:

• この建物は = as for this building
• かつて高校だった = it was once a high school

This does not always translate naturally word-for-word into English, but that is the basic grammar.

かつて means once, formerly, or at one time.

In this sentence, it tells you that the building was a high school in the past, but is not one now.

Nuance-wise, かつて can sound a little more formal or written than some everyday alternatives. A more conversational way to say something similar is:

• この建物は前は高校だった。
• この建物は昔、高校だった。

But かつて is perfectly correct and natural.

Because 高校 is the predicate noun of the sentence.

In Japanese, when a noun is linked to another noun by to be, you often just use:

• A は B だ
• A は B だった

So here:

• この建物は高校だった = This building was a high school

No extra particle is needed between 高校 and だった.

だった is the plain past form of the copula だ.

• だ = is/am/are
• だった = was/were

The polite equivalent is でした.

So:

• この建物はかつて高校だった。 = plain style
• この建物はかつて高校でした。 = polite style

Both are grammatically correct. The sentence you were given is simply in plain style.

Literally, 高校 means high school. It usually refers to the institution, not just the physical structure.

But in a sentence like this, the meaning is that the building used to function as a high school. In natural English, we often still say This building used to be a high school, even though we really mean This building used to house a high school or This building used to serve as a high school.

So the Japanese is natural, even if the logic is slightly shorthand.

Japanese does not have articles like a, an, and the.

So 高校だった can mean:

• was a high school
• sometimes, depending on context, simply was the high school

English requires an article, but Japanese does not.

Yes. 高校 is a shortened form of 高等学校.

• 高等学校 = full form
• 高校 = very common everyday shortened form

In normal conversation, 高校 is much more common.

Yes, it is natural.

It sounds a bit more neutral or slightly formal because of かつて, but there is nothing strange about it. A very everyday version might be:

• この建物は前は高校だった。

That said, the original sentence is completely fine.

Yes. In fact, used to be is often one of the most natural English translations of かつて〜だった in this kind of sentence.

So these English translations are all reasonable:

• This building was once a high school.
• This building used to be a high school.
• This building was formerly a high school.

The Japanese does not force just one of these; the best English choice depends on style.