I saw it at the Lyceum in 97 and at the o2 last weekend, and was discussing this with friends.
JCS assumes the audience know their Bible stories, and in particular this episode in the life in Jesus.
I had very little R.E at school, and didn't to Sunday school, hence I am not ashamed to admit I am pretty clueless.
Hence I've always felt on the backfoot with JCS, as background is not explained, character motivations and context of the story are only loosely referred to, and now both times I've seen it I am left with so many questions like:
Why did XXX do this/that?
Why did XXX think that about XXXX?
making it hard to really understand what the characters are all about ultimately!
Yes, but the motivations and that stuff aren't in the Bible. All you'll get there is a telling of, "X went to Y and did Z." That's really the point of the whole musical: pointing out what the motivations of the characters may have been, according to Tim Rice, with a modern slant on the thinking. So I don't think that's something which would have really been covered in RE at school.
I would have thought the basic stuff about Jesus - getting the disciples together; Mary Magdalene = reformed prostitute; one of his followers - Judas - turns him in; politcal ping-pong between Pharisees, Pilate, Herod; crucifixion... is all general knowledge anyhow, for most people? Don't know that and there's a gap in general education, I would have thought.
Having said that, the "plot" does come over much more clearly in the film version. As I said, in this version I was more watching the spectacle than following the plot anyhow.
The original thing was already a double-decker sandwich - we have the historical events in vision, and a modern interpretation of the thought processes on the soundtrack. What this new production tried to do was add a third layer to the sandwich - a modern updating of the visuals (while leaving the words alone). That's just over-egging the cake a little. And, yes, if this production was your first exposure to the JC story, you ain't gonna know what the frig is going on.