How old are lydia and Abby?

Well, Maisie was 16 years old when this movie was shot, if that's what you meant...

They are 15 and Lydia may have turned 16 by the end of the movie. They mention that in Abbie's honor they have started a Form 5 scholarship. Assuming that the scholarship would be for girls her own age, that would put her at 15 starting Form 5. Like a Sophmore in American high schools. Someone correct me if I have it wrong.

The only question I have about it is if the scholarship was for what would be the next year of school then she might have been only Form 4, or 14.