Haha.
This old problem. Sorry but i've always found this one a little bit funny.
I'm inclined to agree here. Seriously, I thought this was a resurrection of an old thread with this exact title.
Anyway, I think it would be redundant telling you to advise her of how you spend your free time as you seem set on it anyway (And so, choir, this is why God's love is eternal...). So I'll impart my experience on you, the young padawan, not the first time I've said this admittedly

.
What other people think of you, particularly in school, can be really overrated. That said, don't make a huge deal of it. If you keep your explanation fairly blunt, it gets out of the way faster and you two can continue with your lives both the wiser. It's a hobby, not a lifestyle choice, and I've always wondered why people have such a hard time acknowledging it. But that's just me. Chances are she'll be into something just as sad/silly/money-wasting. If she reads a certain Saga-about-sparkly-vampires-that-creates-double-standards*, you already have some leverage.
In my experience, dislike towards the hobby tends to be based on the stereotype of nerdiness and what they see as a waste of money (which isn't even theirs, so go figure). As such, if you don't really fit the bill that dramatically or have a decent reply to how much of a money sink the hobby is (which, if we're honest, it is admittedly. Anyway, asking the heckler how they spend their money tends to shut them up fairly efficiently), there shouldn't be too much of a problem. I remember watching one guy pick on someone else for being a wargamer. It went quite literally like this:
<Guy waiting outside class for teacher to arrive>
<Other guy walks past>
Other guy (using sarcasm): "Oh, where's your Warhammer?"
He said that before picking up a look of satisfaction. Oh yeah, see what I did to that nerd. I totally put him in his place. LOLOLOLOLCOPTER. Quite.
Finally, if things look like they're going to pot, you can try and cool things a little bit:
*Explain that you're helping the economy, because you need a job to support the expenses.
*Spit out the UrbanDictionary definition of Warhammer ("the cure for having money" if you're interested).
*Tell her that you never really grew up, that you have an inner child who frequently yearns for the occasional taste of freedom. Some people find that cute for some reason.
*Cover all the good that it's done to your life (forgetting spending enough money on it to feed an entire third-world country for a month). For me, it's enhanced my knowledge of archaic military tactics, something that helped me top the class in Classics this year, also picking up $50. It can also requires a degree of patience, cognitive ability and maturity. If you can't tell her anything else, explain that the hobby helped you meet me

.
Anyway, hope that helps. However, I wouldn't be surprised if someone with the key to these parts comes around to lock this fairly soon.