Some of the schools I was accepted to said things like:
-Finish with a GPA at or above the GPA you applied with (this one scared me to death)
-Finish at "the same academic level you've been performing at"
-Finish with grades no lower than a "B"
-Finish with grades no lower than a "C"
Those first two I listed were from my top two choices, so I was pretty nervous. I called them and asked how strict they were about their policies, and both said that they just don't want me to give up - that it's not a big deal if I get a few lower grades. Needless to say I was very relieved!
Each school is different. Your best bet - call them and keep regular contact with them. I have heard about students' offers being rescinded because of a D or several C's.