There is not a lot of geometry on the test, but there are a few questions. Yes, it advantages students coming from schools that offer Geometry to middle schoolers but you can get around it by acing every other part of the test. Know that SWW does not curve or take one's limiting school's course offerings into account.
If your child wants to go to SWW enough to do some basic prep, have them learn some Geometry on their own via Khan Academy.