Hey Krista,
There are guns in CustomNPC...you can disable them from firing as the previous posters have mentioned. I am not a big fan of asking mod developers to change how they have made something work, if I can do something about it myself.
I get the feeling though that the problem is not the guns, or the firing...but rather you setting expectations for your students. I have been using this most amazing mod by Noppes for the past 4 years and I rarely have an issue with students and the guns.
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First of all, let them play with them...and get it out of their system. Maybe show them how they can be used to accomplish in game goals. Once they have had a chance to play, it will get it out of their system. We all like to play with new things.
Next, the only time they can get the guns is to craft them, or you put them into creative mode and they take them. You need to set expectations of your students. If they are off task and playing with them, make it clear they are making a choice to not participate in the MC lessons. You would be surprised at how quickly it stops.
Finally, remind them that Minecraft at School is not the same as Minecarft at Home.
Hope that helps...empower your students to make good decisions.
MisterA