I've had this happen a couple of times. The fix that I found was to delete the child (yes, delete--go into buy mode, turn on moveobjects, and delete him/her). The game already considers the child at school when the portrait is greyed out, it just forgot to send them. When you delete the child, the game proceeds as normal. This includes the kid getting the grades and skill points, being able to call and chat or call them home, and graduating as they should.