Forest Lake Christian "Tree" in left center field ground rule

After listening to the coach give the ground rule for the tree in left center field that is in the field of play I kind of shrugged my shoulders and said that doesn't sound right. Upon going home and looking into rule book, I don't think they can have the ground rule that they have.
The ground rule as stated say that the tree which is approximately 50 feet in front of the fence has a line painted on it about 20 feet high. If the ball hits above the line they play it as a Home Run, if anything below the line it is considered live.
My interpretation from the rule book is that no ground rule can supersede a book rule. With this in mind since the tree is in the playing field, any ball hit into it you should be a live ball. You can not have a home run when the ball does not go into dead ball territory and you can't arbitrarily make part of something that is in the field of play dead ball territory.
If anyone else has thougths on this please repsond