I don't think you're being stupid, no. I agree that it's an informal term... I don't know of anyone who would disagree, but maybe that has something to do with where I live. I use y'all and "you guys" (or yinz when I want to be obnoxious) when I want to pluralize "you," but I never use it in a formal context, as a rule.

But I do kind of wish that English had some form of plural "you," like other languages. I feel like we're missing out.
Originally Posted by SCG
"You" actually is the plural one from back in the day, as well as the formal way to address one person. "Thou" was singular and informal. Our language started out with that feature included, but then it got phased out.

...It's a siggie.