Backstory: Mac Gargan entered the Spider-verse as a private investigator. He was hired by J. Jonah Jameson to find out how young Peter Parker was consistently able to get high-quality photos of Spider-Man, though Parker's active spider-sense meant he could easily evade the lousy PI.
Fed up with the poor results, Jameson paid Gargan to undergo a medical process that would give him the characteristics of a scorpion. Gargan gets some Spidey-equalling superpowers, but ends up with the unfortunate side effect of insanity.
If Ifans Played Him: He'd inject a nice amount of humour into the efforts of the bumbling private dick.
Spidey-Sense Tingling? The character has enjoyed a long run in the comics (he later became one of the Venom incarnations), so he could have franchise-building potential.
Key Scene: In order to make it work in the high-school setting, we'd make Gargan a caretaker who gets suspicious of Peter Parker's erratic behaviour.
Marc (500) Days of Summer Webb should be able to come up with a pretty decent sequence in which the hapless caretaker follows a newly-empowered Parker around the school, struggling to keep an eye on him while making it look like he's carrying out odd jobs...