This is a common problem and there are a few things that can be done to help. First of all, the parish should be sponsoring activities for all the children in the parish. These activities should be advertised in the bulletin and not only in flyers sent home with the day school kids. Parish activities such as archdiosen league sports, scouting, choir, bell choir, youth group, should not be limited to the day school kids. Also, the public school parents should volunteer to help.
I think if the adults who have kids in the public school are active in the parish, it is more likely that activities will be set up for all the children as well.
Another thing I strongly believe in is to encourage the priests to attend the major events at the childrens’ school. For example, invite him to watch the children’s orchestra concert, big game, or graduation ceremony. These invitations will be more meaningful if they come from the children. Granted, priests are busy men, but eventually they might make it to see a school play and means alot to the kids to see them there. When I was in school, our priests were often present at our major school events. One of them even offered an opening speech at our baccalaureate ceremony when I graduated.
Also, it wouldn’t be a bad idea for members of the church to attend the local school board meetings. Any taxpayer can attend (if it’s an open session) and even if one doesn’t have children, they definately have a vested interest in how things are going in the public school. (Poor schools mean declining property values) Then, if it should happen that the public school begins to demonstrate an intolerance for the rights of the children of faith that attend there, it wouldn’t be a bad thing for the local priest to be a familiar face to the school board. Believe it or not, it has not been unheard of for a clergyman to sit on a public school board. (Though I don’t know if a Catholic priest would be allowed by his bishop to take up that responsibility given all the other things he has to do.)