Where do you suppose the gum and the pennies are comming from?

This is really a question that you should answer
yourself.  The reason I say that is because everyone who answers questions on here knows
where the gum and pennies are coming from.  So it's hard for us to wonder where they're
coming from.  From the sound of the question, I think your teacher is more interested in
your thinking than in the actual answer -- who actually puts them
there.

Jem and Scout seem to think that they belong to
someone who rides the bus to school.  That person has stashed them and forgotten them. 
This makes sense because who else would hide stuff like that in a
tree?

I guess you might want to consider that question. 
What kind of person would leave stuff in a tree other than a kid?  I suppose it would
have to be someone a bit strange, right?   And think to yourself where the tree is. 
Maybe between those, you can figure out who it is.