You are right, it is better not to tell on these girls. Primarily, this is because the rule is set up in order to help you improve your Chinese, so if they don't care about improving their Chinese then why bother trying to force them to adhere to the rule?

I'd guess the girl you're friendly with, the one who is actually good at Chinese, probably got sucked into speaking in English with the other two through peer pressure. It's sad, but it explains why she didn't really give you a reply.

Think happy thoughts!