Either your boss is not very well informed, or he thinks you are not very well informed (which given the fact that you are new may be true, but not an excuse).
As I'm sure many other will have posted on here, you are entitled to duty free lunch. There are some exceptions, but this is not it. We all work through lunch sometimes, but that is a choice, and there is nothing wrong with saying "yeah I can do it afterlunch, or before school." Bottom line is he does not have the ability to seek repercussions for that.