There's a difference between righteousness through Christ, and "self-righteousness", which is just another term for arrogance. Jesus did say that God wants us to love Him with our minds as well as our hearts, and we can't do that without knowing why we believe and trust in Him. We're commanded to have an answer ready whenever anyone asks us to explain our faith in Christ. The term used for "answer" in this context is the word "apologia", which is where we get the name of "apologetics". Its the equivalent of a courtroom defense, and Christians don't get points for being stupid. Here's a clip from one of my favorite apologists, where he explains why "faith is not enough"...I think people being malicious toward Christians in situations like this comes from a good number of Christians living the "it's us versus them" mentality... and this is coming from me, who has been a Christian for 17 years. I know i used to have that, where I felt I had to constantly show and defend my faith, and the more people protested, the harder I shouted. If I would have worked in that school in my younger years, and I was told to remove my Bible, I'd keep it there and the posters and declare that it was my right and consider it a war against those who would war against God.
I'm not like that anymore, but looking back, I can see why so many people can not only be atheistic, but against Christians in general. Who knows what this teacher has done in the past to rile his bosses, but he obviously wasn't listening to their rules.