i would like to study business with polish in warsaw
In this case I think that SGH would be the best...
sgh.waw.pl
And also are high school results very important in deciding if you get accepted or not or would my previous university results be sufficient?
Since few years the system here is generally that what matters is the number of points got on the final exams (matura) in high school but I don't know how that looks like in case of foreigners or people, who already have BA and are interested in MA, which is probably the point in your case...
Here you have some info and contact...
sgh.waw.pl/ksztalcenie/international?set_language=pl