Kelvin,
The vast majority of libertarians derive property rights as you do, and I used to as well. However, I think there is a much stronger case for libertarian property rights than if you just start at “because I used it first”. While I think first use is basically a good rule, I think the reason why people are attracted to it is neglected by libertarians: we are libertarian because we prefer social cooperation over conflict. First use is one of the best norms for the vast majority of situations for a variety of reasons, though there are other norms that can pick up where first use is inadequate.
Ultimately, what it comes down to (in my opinion) is that people who reject the libertarian view of just property prefer social conflict as a means to ends. In other words, the ends justify the means. If they have to rob a “rich” man (I put rich in quotes because clearly the tax burden in America falls mostly on the middle class and not the elites) in order to pay for “charity”, then clearly they believe that the ends justify the means. It doesn’t matter to them how the “charity” receives the money. It could receive it through social cooperation or conflict, it doesn’t make a difference to them.
Anyway, if you just say to other people that first use is moral because it is, there is no reason why anyone will necessarily agree with you. If you frame the debate around social cooperation against social conflict, then you are in a much stronger position. Your teacher must now defend conflict as a means to ends, whereas you are the one defending cooperation.
Watch him squirm.