High schoolers are already a very connected population. They know who has a car and who doesn't, so I'm not sure they really need a new app to build car pools. Furthermore, there's little incentive for a high schooler with a car to offer up their services to students they aren't already friends with.
I'm not sure how you can incentivize the drivers. Paying them -- making Uber for high schoolers -- sounds like a bad idea, and the rider students aren't exactly flush with cash to pay anyways.
You might find more success trying to make an app that connects parents of students for car pooling? Parents are incentivized by getting someone else to drive their kids to school and can work out non-monetary agreements like 1 family drives Monday, Wednesday, Friday, and the other family drives Tues, Thurs, and to games/events. But again, apart from running ads, it's not clear to me how you would monetize this.