К счастью, пострадавших не было, и тревога оказалась учебной.

К счастью is a very common fixed expression meaning fortunately or luckily.

• к normally takes the dative case
• счастье becomes счастью in the dative singular

So literally, it is something like to happiness, but in real usage it simply means fortunately.

This is an idiomatic expression, so it is best learned as a whole:

• К счастью, ... = Fortunately, ...
• Compare:
  • К сожалению, ... = Unfortunately, ...

Because К счастью is used here as a parenthetical introductory expression, not as a normal part of the sentence structure.

In Russian, introductory words and phrases like these are often set off by commas:

• К счастью, пострадавших не было.
• К сожалению, поезд ушёл.
• По-моему, это неправильно.

So the comma shows that К счастью is commenting on the whole statement, rather than acting as an ordinary sentence member.

Пострадавших comes from пострадавший, which is originally a participle from the verb пострадать (to suffer, to be harmed). But very often it is used like a noun.

So in this sentence, пострадавшие means:

• injured people
• casualties
• those affected

This is very common in Russian: a participle or adjective can function as a noun when the meaning is clear.

For example:

• раненый = a wounded person
• больной = a sick person / patient
• пострадавший = an injured person / victim

Here, пострадавших means injured people / casualties.

Because Russian often uses the genitive in negative existence sentences — especially with не было (there was/were not).

The structure is:

• кого/чего не было = there was/were no ...

So:

• пострадавшие = nominative plural
• пострадавших = genitive plural

That is why the sentence says:

• Пострадавших не было = There were no injured people / no casualties

Compare:

• Люди были = People were there

• Людей не было = There were no people

• Ошибки были = There were errors
• Ошибок не было = There were no errors

This is one of the most important Russian patterns to learn.

Because this is an impersonal existential construction: there was/were not.

In Russian, when you talk about whether something existed or was present, you often use было in the neuter singular, regardless of whether the thing would be singular or plural in English.

So:

• Пострадавших не было = There were no casualties
• not Пострадавшие не были

Why not не были? Because не были would usually mean something more like:

• the injured people were not... and it would need some continuation:
• Пострадавшие не были готовы = The injured people were not ready

But here the meaning is existential:

• there were no injured people So Russian uses:
• не было

Оказалась adds the idea of turned out to be or proved to be.

So:

• Тревога была учебной = The alarm was a drill / was a training alarm
• Тревога оказалась учебной = The alarm turned out to be a drill

The verb оказаться is very common when the real nature of something becomes clear afterward.

Examples:

• Он оказался прав. = He turned out to be right.
• Фильм оказался скучным. = The film turned out to be boring.
• Тревога оказалась учебной. = The alarm turned out to be a drill.

So оказалась gives a stronger sense of discovery or clarification.

Because after оказаться, Russian usually puts the predicate noun or adjective in the instrumental case.

So:

• учебная = nominative feminine singular
• учебной = instrumental feminine singular

Since тревога is feminine singular, the adjective agrees with it in gender and number, but it appears in the instrumental because of the verb оказалась.

This pattern is very common:

• Он оказался врачом. = He turned out to be a doctor.
• Она оказалась умной. = She turned out to be smart.
• Проблема оказалась серьёзной. = The problem turned out to be serious.

So in your sentence:

• тревога оказалась учебной

Not exactly. The adjective учебный / учебная / учебное can mean training, practice, instructional, or for learning depending on context.

With тревога, it means a drill alarm or practice alert, not a real emergency.

So:

• учебная тревога = a drill / practice alarm / training alert

Other examples:

• учебник = textbook
• учебный год = school/academic year
• учебный полёт = training flight
• учебная тревога = practice alarm / drill

So here учебной means the alarm was only a drill.

Yes, Russian word order is flexible, though the original version sounds natural and neutral for news-style reporting:

• К счастью, пострадавших не было, и тревога оказалась учебной.

You could rearrange parts for emphasis, for example:

• Пострадавших, к счастью, не было, и тревога оказалась учебной.
• Тревога оказалась учебной, и, к счастью, пострадавших не было.

But the original order is very typical because it presents the information in a logical sequence:

1. good news first: fortunately
2. main result: no casualties
3. additional clarification: it was only a drill

So yes, other orders are possible, but the given one is natural and stylistically smooth.