Hi Hasib, interesting question. All three sentences are much better with present simple: before the final bell rings. If you use 'would ring,' it sounds a bit strange and I suppose would be considered a hypothetical situation rather than a real situation.

Personally I would use “would ring”, but it’s not something that would be wrong either way and the meaning wouldn’t change very much either way. The difference is whether you are allowing for the idea that the bell might not ring. This is phrased like this is an on going request, has asked vs had asked, so the bell ringing is a future event, something we have limited knowledge of. We expect the bell to ring but the future is not certain, so we can show this uncertainty by saying “ would ring” or ignore it. Either way, it don’t effect heart of sentence, the repeated requests, so it probably doesn’t matter.

See other post. Are they the same? :)