They mentioned that Lucy is 19 and is about to graduate high school. Most kids graduate at 18 unless they are held back for failing. It was never mentioned that Lucy had to repeat her Senior year of high school only that she was skipping school in the First Season of the series.
During the party at Southfork, while Kit and Lucy are talking, Mr. and Mrs. Mainwaring arrive. Lucy tells Kit she wants to go say hello. Lucy greets them at "Wainmaring" instead of Mainwaring.