Re: Are there definite relationships in dimensions between various body parts?

Interesting question.  I don't think that you can show that 

Arm length = Foot length 

or something like that.  However, you may be able to do something similar.  

Let's say you have very tall persons and very short persons.  We can
probably say that the forearms of the tall people are longer than the
forearms of the short people etc.  We can probably say that the same thing
holds for legs, fingers, etc.  This longer-shorter thing leads to a
"correlation coefficient" which means that there is a relationship, but not
probably one of equality.

That is sort of an advanced idea however.  To look at your very specific
question, I would take the kids in your class and measure them.  Then you
can try to look for relationships between them.  I would do the boys and
girls separately.  
To really understand this question, you probably need a little more math
than you have.  If you wish to pursue it, try to find information about
"correlation coefficients" or "Pearson coefficients".  Good luck!!