How the universe started

[Efbe]

So, without being all negative, and 'it's far too hard to explain'

Can anyone give me a good explanation for how the universe started that could be given to highschool children? Essentially I need the bit before the big bang

I have tried this myself and failed badly, through basically not knowing the subject well enough.

[Efbe]

Thanks all. Very useful stuff here.

I have listened and read some Hawking which has given me a few more things to say on the subject; though generally speaking the big bang concept is quite well known by high school children.

The problem is they always ask the question; so what was there before, where did the bang come from?

Now I get it's completely theoretical, and we can't prove any of it, ergo we will only have a hypothesis.
From reading Hawkings stuff I got the impression the answer was likely to fall in the way time works/flows; so if not a contraction/expansion theory, then a time goes in a loop, or moves backwards and forwards, and so there was no start, just a loop.

So can I change my question slightly:

What's the most most prevalent/agreed upon theory of what came before the big bang?