Thanks for your question. Here is an overview of the ADHD diagnosis process, and here is a sharepost from expert Eileen Bailey listing the people who can diagnose ADHD.
Doctors and others who diagnose ADHD often rely on information from the school to paint a picture of what the child is like in the school setting, to see if he or she shows symptoms. Perhaps that is what your pediatrician means by saying he or she wants testing done at school. Whatever the case, you may want to clear up what he meant by "getting tested at school" and why wants it.
I hope this helps.