Every move you make in life has a tremendous toll on your future; high school is not the only place and time where your actions have bearing. While there is no denying many high school relationships perish, a handful continue through college and into adulthood. And even if it doesn't, even if your relationship ends with high school, so what? You've gained a whole new experience you would have otherwise missed and can learn from mistakes in the past and prevent them repeating in the future.
What worth is a few extra percent in an exam in the grand scheme of things? No one cares whether you got 99% or 90% when you have an A*, and certainly once you graduate from university no one will care about your A*s at all. You have one chance at being 18; don't waste it.