Nope, they depend on how well you do later in the year. There was this one kid at our school that got into Princeton, totally didn't try for the rest of the year, and ended up going to our local county college because Princeton took away his acceptance around June, way too late to apply to any other colleges.
Your B though, don't worry about it. If you were addmitted having straight As I'm sure it still won't make a difference. As long as you don't drop two grades in your final average, in my opinion, I think you're safe. At Boston College, they say that the lowest is a C as a final average for senior year, or else they'll start looking closer at your grades.