More than friends

I've been friends with this girl for a long time, and we've always been in class 1 according to this version of the DiCarlo Escalation Ladder, but I am there with most of my female friends so I've never thought of it that way. However we were both at a party a few days ago and got to class two no problem in the hot tub, with other people in there as well and none of them were couples. Of all the people in the hot tub we definitely stood out as having the arms constantly around each other and what not, and in fact the party was pretty tame to be honest. The party ended and that was the end of that and nothing else has really happened since then.

I'm not really sure how to proceed because I find that we always have a lot more fun in person that via texting or any other online form of communication. I'm just not sure how to plan something with intentions of going somewhere more than friends because in the past we've had 1 on 1's where all we do is just hangout.

ps. we are in high school