This is not my kind of thing, mostly because I am not a teenager, but maybe you have a teenager, or a much better body than me.
Anyway, apparently all day dresses are currently Buy One, Get One Free at Rampage. The discount is automatic at checkout. Even the ones on sale qualify, so you may be able to score quite a nice deal. I require a bit more coverage than their clothes seem to offer, but I don’t judge. Have at it.