A public school trumpet teacher mistakes a 15 year old school girl for her trumpet: http://tinyurl.com/nawcwz

In a legal sense, it's very illegal. I don't think it's morally unsound. If the girl is mature enough to recognize that she is a member of that particular league table (and comprehend the intricate, knotty folds of cunt and sexuality), then she's fair game.
Having said that, she may have been coerced by the elder to blow the forbidden brass without properly acknowledging the unusual procedure of sexual conquest.