Information wanted on Freemason Cornerstones...?

It is very interesting to me that the public SCHOOLS in my area have the Freemason cornerstones and they all have "time capsules" behind the plaque. I live in South Florida and I was surprised to see them. I was curious if anyone has any idea how the Freemasons are able to put such a cornerstone on public schools? Why would they be involved with such a building? I do know and understand the Freemasons. I am very surprised that such a group with religious affiliations are able to be part of our public school buildings.