1. the rumour isn't "Tokiwadai can take over the world", it's "Tokiwadai can take over the White House". Which, if they ever plan to do that, they definitely can. Unless Touma is somehow conveniently there when the takeover happens, I don't see how a bunch of soldiers with just guns can stop all the OP girls. And like, sure nuke will solve the problem, but that would require them to nuke the god damn White House too.
2. Magicians are basically myths in Academy City, people don't believe in them, so of course they won't think about them when creating that rumour.