Yeah, the advice of having more than one game available is sound. Obviously if some kids say they want to play Malifaux, you don't turn them away. However, I can say from my own experience of being a kid of that age, trying out a similar club and still knowing many kids that age that, if you do start a wargaming club it will pretty much end up being a
40k club anyway - that is by far the most common system played at that sort of age and level.
In terms of getting people interested, don't let the price thing worry you. Sure, large 1850pt armies will be costly, but if you start with and encourage games in the 500-750 pt range, all the new kids are going to need is the codex, one of
GW's new bundles and maybe a
HQ. That is probably well within a Christmas or Birthday budget for most parents. It's also worth having a "starter set". See if you can get Stormclaw off eBay but, failing that, see if two people are willing to leave some models aside at club nights along with an open rulebook, for any complete beginners to try.
As far as getting the club going - you will need, first and foremost, a teacher to "run" it. All he/she may do is sit around, but your school will require a teacher to be present. Similarly, you need to find a place and time that your school is happy with. You'll likely need to get a few people to put their names down expressing interest as well, otherwise the school aren't going to run with it. Then you need to get your hands on a bit of scenery (in my experience - school kids really aren't that bothered about it being that numerous or high quality, but it's nice to have the odd building or forest). If there's a few kids who are keen, get together and have a scenery-making session. After that, just advertise and, hopefully, it'll all go smoothly.
Good luck
