Когда прозвучит звонок, урок закончится.

Because Russian usually uses the future in both parts when it is talking about a future event.

In English, we say:

• When the bell rings, the lesson will end.

So English uses a present form (rings) after when.
Russian does not do that here. Russian says, literally:

• When the bell will ring, the lesson will end.

That is why you get:

• Когда прозвучит звонок, урок закончится.

This is completely normal Russian grammar.

They are perfective because the sentence refers to single completed events:

• the bell will sound / ring once
• the lesson will come to an end

Perfective verbs in Russian often express:

• a one-time event
• a completed result
• a change from one state to another

Here:

• прозвучит = will sound / will ring out
• закончится = will end / will be over

If you used imperfective forms instead, the meaning would change and would usually suggest an ongoing, repeated, or habitual action.

Because звучит is imperfective and usually means is sounding or sounds in a general/habitual sense.

Compare:

• Когда прозвучит звонок, урок закончится.
  = When the bell rings, the lesson will end.
  A specific future event.

• Когда звучит звонок, дети выходят.
  = When the bell rings / whenever the bell rings, the children leave.
  A habitual or general situation.

So in your sentence, прозвучит is used because it points to one future moment.

Прозвучит is the 3rd person singular future form of прозвучать.

In this sentence it means something like:

• will sound
• will ring out
• will be heard

With звонок, it naturally means:

• the bell will ring

A very close alternative in everyday Russian is прозвенит звонок, which many learners also see. Both are possible, though прозвенит is especially common for a bell.

In закончится, the -ся is part of the verb закончиться.

Here it does not mean a true reflexive idea like himself or itself.
Instead, it helps form a verb meaning:

• to end
• to come to an end
• to be over

So:

• урок закончится = the lesson will end

Compare:

• закончить урок = to finish the lesson
  someone finishes something
• урок закончится = the lesson will end
  the lesson itself comes to an end

Because both are the subjects of their clauses.

• Когда прозвучит звонок
  звонок is the thing that will sound
• урок закончится
  урок is the thing that will end

Subjects are normally in the nominative case in Russian.

Because Когда прозвучит звонок is a subordinate clause, and in Russian subordinate clauses are normally separated by a comma.

So:

• Когда прозвучит звонок, урок закончится.

This is true even when English might feel lighter or more flexible with punctuation. In Russian, the comma here is standard and expected.

Yes. You can also say:

• Урок закончится, когда прозвучит звонок.

This means the same thing: The lesson will end when the bell rings.

The difference is mostly about focus:

• Когда прозвучит звонок, урок закончится.
  starts with the time condition
• Урок закончится, когда прозвучит звонок.
  starts with the main statement

Both are natural.

Russian has no articles like a or the.

So nouns like звонок and урок can mean:

• a bell / the bell
• a lesson / the lesson

The exact meaning comes from context. In this sentence, the context clearly makes them definite:

• the bell
• the lesson

Yes. Урок can mean:

• a lesson
• a class period

So depending on context, the sentence could be understood as:

• When the bell rings, the lesson will end
• When the bell rings, class will end

In a school setting, both are very natural interpretations.

It can work for either, depending on context, but the most basic reading is a specific future situation:

• When the bell rings, the lesson will end.

Because both verbs are perfective future, it sounds very natural for a particular upcoming event.

However, Russian also often uses this structure for something that is generally true in a repeated setting, especially if the speaker is talking about the usual sequence of events at school. The grammar itself does not force only one interpretation; context decides.

Yes, урок кончится is possible and means almost the same thing.

• урок закончится
• урок кончится

Both mean the lesson will end.

But закончится is often felt to be a little more neutral or standard in many contexts. For learners, закончится is a very safe choice.