Private schools can be more demanding on students than public schools. If the work load is really too much you could talk to your parents about switching schools.
If you don't want to switch schools, but still can't deal with all the work, talk with your guidance counselor about switching to easier or lighter classes.
It doesn't sound like you're having a problem understanding the work, it just sounds like you don't want to put so much time into it. If you have plans to go to college when you graduate, though, a heavier work-load in high school will help to better prepare you for college.
There are always weekends to hang out with friends, and maybe you could set a schedule that you go out one or 2 nights a week instead of every night.
Think about your future, and what you feel is more important in the long run and decide what to do from there.