Learn Hardcore Indonesian: Saya tidak tahu seberapa lama rapat sekolah akan berlangsung. - I don't know how long the school meeting will last.

berapa lama is the more common and neutral way to say how long (duration).
seberapa lama can add a nuance of to what extent / how long exactly / how long after all, giving a slightly more “open‑ended” or “vague degree” feeling.

In everyday speech, most people would probably say:

Saya tidak tahu berapa lama rapat sekolah akan berlangsung.

Using seberapa lama is still natural; it just sounds a bit more like you’re emphasizing the uncertainty of the length.

Where does seberapa lama go in the sentence? Can I move it to the end?

In the original sentence, seberapa lama comes before the clause it refers to:

Saya tidak tahu seberapa lama [rapat sekolah akan berlangsung].

You can also move it to the end of the embedded clause, especially in spoken Indonesian:

Saya tidak tahu rapat sekolah akan berlangsung seberapa lama.

This is understood and used in conversation, though the original order (seberapa lama rapat sekolah akan berlangsung) sounds a bit more neutral/formal and is safer for writing.

Is akan required here? Does it mark future tense?

akan is a modal that usually indicates future or intended actions, similar to will or going to.

With akan:
rapat sekolah akan berlangsung → the school meeting will take place / will last
Without akan:
rapat sekolah berlangsung berapa lama? → can be how long does the school meeting last? (habitual) or how long is the school meeting (lasting)?, depending on context.

In your sentence, akan is natural because the meeting is in the future or not yet finished. You could say:

Saya tidak tahu berapa lama rapat sekolah akan berlangsung. (very natural)
Saya tidak tahu berapa lama rapat sekolah berlangsung. (also possible; context decides if it’s future or general/habitual)

So akan is not strictly required by grammar, but it makes the future time reference clear.

What exactly does berlangsung mean, and are there other verbs I could use?

berlangsung means to take place, to go on, to be in progress, to last (in time).

In event contexts (meetings, ceremonies, classes), it’s very common:

Rapat akan berlangsung selama dua jam.
The meeting will last for two hours.

Alternatives (with slightly different nuances):

diadakan – to be held (focus on the event being organized):
Rapat sekolah akan diadakan besok.
berjalan – to run/go (informal for events, processes):
Rapat sekolah berjalan cukup lama.
Using a time‑cost verb:
Saya tidak tahu rapat sekolah itu akan memakan waktu berapa lama.
(I don’t know how much time the school meeting will take.)

But for a direct equivalent of will last, berlangsung is a very good choice.

Does rapat sekolah mean “school meeting” or “meeting at the school”? Why no preposition like di?

rapat sekolah is a noun + noun phrase:

rapat = meeting
sekolah = school

Together, rapat sekolah usually means a meeting related to / of the school (e.g., teachers’ meeting, parents–teachers meeting), not just any meeting physically located at the school.

If you want to say a meeting at the school (location), you normally add di:

rapat di sekolah = a meeting at school

So:

rapat sekolah → meeting that belongs to or is about the school
rapat di sekolah → meeting whose place is at the school

Can I drop Saya and just say Tidak tahu seberapa lama rapat sekolah akan berlangsung?

In casual spoken Indonesian, people often drop Saya (I) when it’s clear from context:

(Saya) tidak tahu seberapa lama rapat sekolah akan berlangsung.

Listeners will usually understand that Saya is implied.

However:

In writing, especially formal or neutral contexts, including Saya is clearer and more standard.
If there is any chance of confusion about who doesn’t know, keep Saya.

So it’s acceptable in casual speech to drop it, but Saya is the safest choice for learners.

What is the difference between tidak tahu, nggak tahu, and tak tahu?

All three can mean don’t know, but they differ in formality and style:

tidak tahu – neutral and standard; fine in both spoken and written Indonesian.
nggak tahu / gak tahu – informal, colloquial; common in everyday speech, texting, chats.
tak tahu – more formal/literary or regional flavor (common in some Malay varieties and in written style; also used in some Indonesian dialects).

Examples:

Saya tidak tahu seberapa lama rapat sekolah akan berlangsung. (neutral)
Aku nggak tahu seberapa lama rapat sekolah bakal berlangsung. (informal, spoken)
Saya tak tahu berapa lama rapat sekolah akan berlangsung. (slightly more literary/old‑fashioned in Indonesian)

For learning standard Indonesian, tidak tahu is the safest default.

Is this sentence a question inside a statement? Why don’t we need apakah or a question mark?

Yes, it contains an indirect question (an embedded question):

Direct question:
Berapa lama rapat sekolah akan berlangsung? → How long will the school meeting last?
Embedded in a statement:
Saya tidak tahu berapa lama rapat sekolah akan berlangsung.
I don’t know how long the school meeting will last.

In Indonesian:

You do not use apakah with berapa / seberapa in this kind of embedded question.
You do not use a question mark, because the whole sentence is a statement, not a direct question.

So Saya tidak tahu seberapa lama rapat sekolah akan berlangsung. is correctly punctuated as a statement.

Are there simpler or alternative ways to express the same idea?

Yes, several natural variants are possible, with slightly different structures:

Replace seberapa with berapa (very common):
- Saya tidak tahu berapa lama rapat sekolah akan berlangsung.
Move the berapa lama phrase:
- Saya tidak tahu rapat sekolah akan berlangsung berapa lama. (more spoken)
Use a noun form lamanya (the length/duration):
- Saya tidak tahu lamanya rapat sekolah.
Use a “time cost” expression:
- Saya tidak tahu rapat sekolah itu akan memakan waktu berapa lama.

All of these can express roughly the same meaning as the original sentence, with small nuances in style and focus.