Will graduate school rescind my offer? I am currently a senior in college who got accepted into a math PhD program in the US. This semester I slack off a little bit and as a result, I might get a B and B- in two math courses (I got an A in the third math class). Is it possible that the school will rescind my offer, given that the two courses that I got a B and B- are math courses?
No.

The rules may allow rescinding an offer, but they will not do it.  The program needs to look after its reputation.  A student who got the occasional B is normal.  Rescinding an offer is not.