I think it matters to the school in the sense that lead poisoning is bad, it's good public policy to have every small child tested, and this is a way to make it happen. Also, since lead poisoning can lead to behavior problems, learning delays, etc. I guess it would effect the teachers dealing with the child.
Also, as the parent, if I were to send my child to school (or daycare) I would want a level taken before they started, so that if they had problems later and testing showed a high level I would know the source was the school and not my home.
But if it's really an issue for you you probably could just write your own letter, but I can't find any sort of info on actual law allowing for exemption. I guesss blood draws could be against your religion? But would you not get one in other circumstances?
Previous thread-lead testing exemptions for MA