I was walking to my next class and all of a sudden two black girls start yelling cracker to every white kid they see. I was like I wasn't offended but it got me thinking if it's considered offensive?

if u take offense at it, it's offensive
it's not racist though

Yes, it can be, but it shouldn't be compares to slurs against racial minorities/people of color. There's a huge difference there.

Yes. Never heard anyone use it though.