The start of opening theory

Once new players get over their obsession with Scholars Mate, they usually move onto to more sensible openings. However it is often still 'make it up as you go' lines, so they still walk into some well known traps.
One obvious variation (which I witnessed this very morning) is 1.e4 e5 2.Nf3 Nc6 3.Bc4 Nf6 4.Ng5 d5 5.exd (So far so good) Nxd5!?! Now normally this is considered bad, but at school level Black often gets away with it. This is because at this level, neither player really knows opening theory, so moves like 6.Nxf7 or even 6.d4! aren't that easy to find. As I said to my class this morning, a move like Nxd5 is only bad if your opponent has read that Nxf7 is good.
Of course once most players start learning a bit about openings, then the refutation of 5. ... Nxd5 soon comes up. Certainly it was one of he first things I learnt regarding openings, and I suspect most older opening books have this towards the front (normally quoting 400 year old analysis). Every now and then I come across an article which suggests that the sacrifice on f7 (with or without d4 first) isn't so clear cut for White, but if this was the case I would expect to see a lot more captures on d5 in higher level games. As I have not observed a tsunami of Two Knights games, then I will continue to believe it is more wishful thinking than cold hard analysis.