worried about grades

That's an issue. I have been on my house's scholarship committee for two years and what we have found is that grades don't mean much because so many schools hand out grades like candy. The standard is that you get an A unless you screw up because otherwise mommy and daddy would call to complain. That's especially true at private schools where those large checks are supposed to guarantee A's. So we put more emphasis on ACTs because that's a standard, uniform measure. If you want to get into a top house try taking the ACTs again and this time study for them.

Your GPA isn't an issue. My advice is that if you can take the ACT again, do it. Not only will it help you with sororities but you can get better scholarships. Maybe take some classes and aim for around the upper twenty to lower thirty range, it seems like a reasonable goal given your current score.

That being said, in my house, if you make the grade cut we don't worry about ti after the first round. If you're invited back to any house after the first round it means you've passed their grade cut and it's no longer an issue, you won't be cut on grades from that point on unless they find out you were lying about it before.