Talk with the school counselor and principal about your concerns, and ask what is being planned to help the children with these changes. Since the school is closing, it will be very appropriate to have some kind of celebration of the school at the end of the year. A program to recognize the part the school has played in the life of the community could involve both current and former students. A "time capsule" could be buried with items that recognize important events of the past year. You could offer to help the staff or the PTA plan the celebration.
The children who are moving may experience some problems with nervousness, anxiety, and fear about the new situation. Be calm, understanding, and supportive of them as they make the change. Ask if the children can be taken as a group before school ends this year to visit the new school they will attend next year. You may want to visit the school grounds over the summer and at open house in the fall to help the children feel comfortable in the new setting.
Ask if there is something each child who is leaving the school could take to the new school, such as a construction paper "brick" or something else that represents the old school. This year, the bricks could be assembled against a wall in the new building, and next year, once the children are attending the school, they could add more "bricks" in a special remembrance ceremony.
Finally, talk with the principal and counselor at the new school and make sure they are making plans to help the new students feel comfortable and included. This all needs to happen again when more children are moved at the end of next year.