I know plenty of Italian teens know English. All of my friends know at least a fair amount. That doesn't make me comfortable to teach them more. Things have to be explained in their native language.
Grammar and so on is best taught in the language you're trying to teach itself. (And when teaching modern languages it's always advisable to be very sparse with superfluous grammatical information. Most people pick up the basics easily enough when you immerse them deeply enoough in the language as it's spoken and written.)