My solution is to wear those close-fitting tracksuit trousers you can find in sports stores (for running) - they let your teacher see your knees without being quite as sheer as tights. (I like them because they're stretchy like tights but they've got a bit more room in them, & they leave a little more to the imagination.)
However, if you're serious about ballet the 'uniform' is something you just have to wear whether your girlfriend likes it or not. And it sounds like you started young enough to consider a career in ballet, in which case don't let a girlfriend (who you might not be seeing this time next year anyway) do anything to deter you. (I'm 'old', and a beginner, so I can get away with wearing unsuitable clothes for ballet - you can't ;-) )
To be honest, Steven, if your new girlfriend doesn't like what you do & the way you are - find another one! (A guy in a ballet class shouldn't have any trouble finding girlfriends anyway!)
All the best
Anthony[/i]