I am surprised there is a not a communication between the school district and you - initiated by the school district - to request this parking area, and to discuss how much land they would like to have available.  Yes, you are expecting it and willing to provide it. However, I think that courtesy call each year would be appreciated by you.  If they are not having enough space, they should be asking you in advance if more space is available. 

The lack of such a phone call/ communication says to me that they are taking it for granted.