Should I kneel or stand for mass in a gym?

At my Catholic school, we have mass in the gym. During the mass, when we are to kneel, everyone is standing. When I looked up this situation on catholic answers I found this article in which Jim Blackburn said that even if there are no kneelers we are still suppose to kneel.
catholic.com/quickquestions/should-we-kneel-at-mass
I asked three priests at my parish. They all said it is okay to stand if there are no kneelers in contradiction to what the GIRM and catholic answers said. Last week my school had a mass, during the mass I was not kneeling during all the times that I am suppose to. I want to mention this confession because this bothers my conscience. How can I confess this to a priest when they don’t even think it is a sin? What do I do for my school’s masses? Do I stand like everyone else or do I kneel?

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May I kneel at Mass in the gym?

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