Ah, the old "slippery slope" argument. Just one example of the many logical fallacies they use.
You had already decided that attending a team function would be acceptable to you, but your study leader tried to undermine your own conscience by imposing his idea of what you should do. Why even have your own conscience, if all you need to do is slavishly follow a bunch of rules? Be advised that if you formally join this religion your life will be all about just that. If you later decide you do not want to follow these rules you will be subject to being disfellowshipped and shunned by everyone you know. Celebrating Christmas is a serious offence in this group.
This organization likes to pretend you can follow your own conscience, but they only allow that in really minor situations, and even then people in the group tend to nitpick and gossip about it.
Really, whether you celebrate holidays is not that important. What is more important is this organization's determination to control every aspect of their members lives. Is that something you can live with? Even more concerning is their continued teaching that Armageddon is coming any minute, something they have been claiming for over a hundred years. They have falsely predicted dates for the end many, many times, so far they have a 100% failure rate on predictions. Why believe them now? Because world conditions are so bad? Every generation believed that.
I was a JW back in the seventies. I was told in the Awake magazine that a young person, (which I was at the time) would never have time for a career in this system of things. I believed that and skipped college, which I now regret, as I am old enough to collect social security now.
I would hate for anyone else to fall for this, so please investigate this religion thoroughly before you commit. Don't believe the line that Satan is trying to receive you and that only information from the Watchtower can be trusted. That's what organizations do when they have something to hide, right? You seem like you have a good head on your shoulders, trust your instincts and don't let others manipulate you, because that it's the true "slippery slope".