She actually hadn't forgotten Roy sneaking onto the Girl's floor and was planning on pounding him when she first saw him the morning, but after hearing earlier from Toni that he did it to give her flowers, she decided to let it slide and not pound him...this time.
She corrects Kitty about the curfew when she misspeaks the time, "It’s 5:58pm, actually, giving everyone even less time. You’re plan’s not feasible, Roy. Even if you could get Nightwing to agree, which seems unlikely to me, there simply isn’t enough time to organize a party with outsiders here. The other kids can’t fly and will have to deal with a traffic rush as everyone tries to beat the curfew. That leaves hardly anytime for the actual party itself. It’s better for those of us allowed to leave to go to an existing party."
She then suggests, "What we should do while we’re out is to spend the last 30 minutes or so grabbing party supplies and, when we get back and the curfew is in place, we can then have our own private party with just us and any of the personnel living here on campus that want to join us. Besides being more feasible, it has the side benefit of helping the staff better warm up to us and improving our relations with them."