Seems a young priest teaching catechism in school heard kids saying siema, nara and spoko and wanted to sound cool and trendy himself. So when he entered the classroom he would greet the kids with a rousing POCHWA! (short for Niech będzie Pochwalony Jezus Chrystus).
OMG. You really believe that's a true story? It's a joke that's about 15 years old. Yeah, kids have been using siema, nara, spoko and the like for at least that long. I use spoko quite a lot when I'm in an informal setting. Actually, I use quite a number of slang expressions. It's probably because I'm not a wholesome innocent lass ;-)
They like to say 'siemanko' instead of 'good morning' at register time. I understand that this is slang for 'hello', but is it disrespectful? I like them to speak 'Polish', but don't want them to think I am an idiot!"
I would say they are trying to be "funny". Tell them firmly that you know what it means and to cut it out. It's waaay too informal and relaxed for a class setting.