Is this correct?

Is the following sentence grammatically correct?

ベルが鳴れば、教室を離れてもいいです。

If you mean "if (or when) the bell rings, you can leave the classroom", yes, it is correct.

ベルが鳴れば、教室を離れてもいいです。
= If bells ring, you can leave the classroom.
ベルが鳴ったら、教室を離れてもいいです。
= Bells will ring, then you can leave the classroom.
[When bells ring, you can leave the classroom.]
.... The latter example sounds more natural because:
- 鳴れば indicates that the speaker is focusing on the condition; 鳴ったら implies "and then."
- The speaker expects the bells to ring, doesn't expect the condition if bells don't ring.

(To a friend now coming)
駅に着けば電話してください。
= If you get to the station, please make a phone call.
駅に着いたら電話してください。
= When you get to the station, please make a phone call.
.... The former one sounds weird, because the speaker must be expecting this friend to get to the station safely.

家に広い庭があれば / あったらいいのになあ。
= I wish I were having a large yard around my house.
.... いいのに clearly shows that this is what the speaker wishes, so either あれば or あったら will do.

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ベルが鳴れば、教室を離れてもいいです。

If you mean "When the bell rings, you may leave the classroom," it is grammatically OK, but the more natural choice would be:

ベルが鳴ったら、教室を離れてもいいです。
(When the bell has rung, you may...)

If you think about it, the bells has to have rung (the action done) before you can leave the room.

This distinction between "...(re)ba" and "...tara" is difficult to explain.... :(

Thanks for the replies.

So, am I right to say that 「ベルが鳴れば、教室を離れてもいいです。」 is grammatically correct though たら would be a much better choice here?

Also, can anybody explain the difference between 「日本へ行けば、富士山が見られます。」 & 「日本へ行ったら、富士山が見られます。」? The first one sounds more natural to me though both are grammatically correct...

In the case of Mt. Fuji example, both sounds fine. Because the two meanings below overlap.

"....(ke)ba" is used to describe a natural consequence - so "if you do A/if A happens, then B of course happens.

"...tara" is used more to desribe "if/when you've done A, then B can be done/B happens," more like a conditional.

These ば and たら are almost the same, and the distinction between them is difficult to explain, as AK says.

The latter たら is a particle, and originally is an auxiliary verb of completion. This たら implies: if the deed / the status as a condition is completed, then.... So, this condition is a sufficient condition.
In that sense, たら is more conditional. たらmay make a weird expression when used for a situation where another condition is required. Your example 「日本へ行ったら、富士山が見られます。」 seems to be one of this case. This example implies: you can see Mt. Fuji only by visiting Japan. So, it’s impudent for the speaker to say so even though the speaker thinks that visiting Japan is not the only condition to see Mt. Fuji. This たらhas a variety of meanings, and showing a sufficient condition is their feature in common.

The former ば is a particle meaning “if” and not so conditional. This ば is used both for a necessary condition and for a sufficient condition (which leads to a natural consequence). It’s not so impudent if someone says 「日本へ行けば、富士山が見られます。」 judging only from the fact that Mt. Fuji is located in Japan, believing that visiting Japan is the only condition or, knowing that there may be other minor conditions required to see Mt. Fuji.

Since the ancient time when verbs of Japanese had a form for subjunctive mood (e.g. If I were a bird,..), ば has been used in both subjunctive and indicative (e.g. If you hope so,...) moods. But, practical distinction between the two moods vanished, and now in modern Japanese ば is not decisively conditional. たら is more colloquial than ば, and its use as a conditional particle is more modern than that of ば. In official documents such as laws, there are found expressions to show a condition without ば or たら.

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Thank you for all the replies. I'm having a much clearer picture now. Thanks again.