Just sit in the back and don't sing along or make the responses. I don't think acknowledging another person during the sign of peace instead of ignoring them would be a sin, if it is not already a sin to simply attend at all. If you think it is valid (you say "doubtful") you may consider praying something like, "Oh Jesus, if you are truly present I wish to adore you and make reparation for the sins of sacrilege committed during this Novus Ordo service."
And so you know where I am coming from, I think the Novus Ordo is valid but a joke compared to the old Mass so I do not go for the most part and if I rarely do (like you do at school sometimes.) I do not actively participate. At the sign of peace I do not go out of my way to shake hands but if someone forces themselves on me I will shake their hand. I don't know where this puts me, probably on the liberal side of this forum on that issue. I try not to reject the entire Novus Ordo Church as I am not a sedevacantist, and I hope there are many Catholics inside of it (I don't know where, perhaps in the Eastern rites, the traditional groups like the SSPX and the FSSP and ICKSP and in Africa and China, perhaps?). Oh and I regularly go to an SSPX Mass (but I am not against the resistance), but as I think the Novus Ordo is valid I go to Novus Ordo Churches to visit Jesus when there is no Mass going on.