Sorry, your comparison is not correct. You're assuming your conclusion when you say "Birds that are not black are also not schoolbuses". In THAT case, it's a perfectly acceptable conclusion to assume, because we can SEE that. But you can't generalize to every other case.
In your example, we may accept the proposition that "If birds that are black are not school buses than this other bird that is black is not a school bus" as true. But we're not yet evaluating the sentence "birds that are black are not school buses". We're only evaluating the if-then statement. So when we look at your secondary example, "If this bird that is black is not a school bus, then birds that are black is not a school bus"... that's not guaranteed by the first if-then statement. Perhaps there's a black bird that IS a school bus (yes, I know, it seems absurd, but, say we're talking to an alien who knows nothing about black birds or school buses... we have to PROVE that). The alien might well accept your first if-then (because it is logically true whether you know anything about black birds or school buses), but you haven't given him any evidence about the second.
"If X then Y" means just that, "if X is true, then Y is true." It doesn't mean that if Y is true, X is true. Y can be true without X being true. Sure, there may be values of X and Y for which when Y is true, X is true, but it's not guaranteed by the first statement. If and only if makes this connection (you may still have to prove the validity of the "if and only if statement" of course, but if you provisionally accept it you can make always deductions about whether X is true based on the state of Y, and vice versa... with an if-then statement you can only make deductions about Y when X is true, and if Y happens to be not true, we can deduce that X is not true, but that's all, at least not without bringing in outside evidence).