"Eavesdropping" is not really the appropriate term even though it's common...it is more like "overhearing" or "listening in". There is no special "right to privacy' when speaking to other people in public. As Ed Ucation points out above eavesdroppping was traditionally punished because it was in fact a violation of the property rights of others. As the OP and the others were in the common ground of the school in which they all had a right to be, there was no eavesdropping or "spying", as Granger refers to it.
Whatever privacy you have is however you construct it. If you want your conversation more private, whisper. If you want it more private than that, get out of the listening range of others. Even more private, pass notes, etc.
The fact that the OP was able to overhear the convo proves that they did not feel the contents of their conversation was worth hiding, and their reaction to his joining the conversation further speaks to that.
Do you never talk to strangers, Granger?
*Advancing the Ideas of Liberty Daily*
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