Well, this could go a number of routes:
(1) The girls decide to not intervene; meanwhile, 'Toro and Paisen have a calm, quiet bath facing away from each other (or with their towels covering them, which has become the usual conduct in actual mixed-baths in Japan), without much happening besides a heart-to-heart chat; with perhaps a few attempts at peeking at each other and some ensuing teasing... although this could wind up in a "you show me yours, I'll show you mine" situation;
(2) Paisen takes the opportunity provided by the setting, the situation and the atmosphere to confess (or attempt to confess) his love to 'Toro;
(3) The girls decide to barge in and break up the situation--which would honestly be the smartest thing to do, considering that this IS a school trip, and both the school and the chaperones could be held liable for students' conduct; this could also be very much a moment-killer for a confession attempt from Paisen;
(4) The girls wind up spying quietly on the pair, refraining from interrupting them in order to see how the situation plays out.
All bets are off if:
(a) The adults (Sana and the 'Toro siblings) get wind of the pair having gone to the onsen together; or if any of them decide to visit the onsen on their own without knowing Hayase and Paisen are already there; or
(b) If Shikki and her two friends wind up either going to the onsen on their own, or also get wind of the pair going down there together. As we've seen from both the "mix-up-in-the-shower" arc and the arc where 'Toro and Paisen visit the love shrine, Shikki herself seems supportive of 'Toro and Paisen getting together, and while she has something of a mild, judo-based rivalry with 'Toro, she doesn't consider her an enemy any more than Orihara does. But her two friends are a totally different story... in fact, the one girl friend of Shikki's (the sniffer) seems to outright hate boys. And as we've seen several times since their introduction, those two aren't too fond of 'Toro, either...
However, don't get your hopes up on our pair doing the nasty (at least not yet). Apparently Nanashi has made it clear he's not going to subject the characters in this manga to any of the ero/hentai stuff he's also known for. And if they do ever get it on, it'll probably just be implied in one of the final chapters or in an epilogue if/when they're married.