Snapedom

Now I'm thinking that maybe Snape did invent Sectumsempra, but Lupin and the Marauders didn't realize that he did.

This is perhaps the most logical and plausible explanation of the Sectumsempra issue I've come across. It does have a certain "feel" to it (at least for me) that corresponds to Severus - that keen, slashing, yet completely controlled way of eviscerating someone before they realize it, that can also be used just for a little 'warning' (James in the Pensieve). Very quick and clean and sharp, you know what I mean? Also the 'severed forever' meaning, the hint at wounds that don't heal quickly (or ever)....

And you're completely right when you mention that we don't encounter other students creating spells (in fact, how many other people can we confirm DO create their own spells?) Yet we know that he's created other spells, and I find it highly likely that he'd use one of his own creations as his 'specialty' rather than rely on someone else's work, considering his whole 'I'm not a passive victim, I'll take care of myself' attitude, and his later emphasis on self-sufficiency.