Teen brain development and patterns are such that many teenagers just aren't able to fall asleep before 11pm or midnight. This all comes down to brain chemistry and when those chemicals are released during a typical day.
So late sleep times, combined with typical high school start times as early as 7:30, means that most teenagers just aren't getting enough sleep. That has all sorts of negative effects on memory, physical development, decision making, and other important things. But most of these things just don't have a big impact on a teenager's decision to try to go to bed earlier.
Finally, here is some research that might actually impact your teenagers to try to go to bed earlier.
It's been clear for a couple of years that adults who get less sleep tend to have higher Body Mass Indexes (BMI), but now it also appears that children who get less sleep in the 3rd and 6th grades are likely to have a higher BMI in 6th grade. Since this finding has held true on both ends of adolescence, my smart money is on it being true in adolescence as well.
But either way, most teenagers legitimately have a hard time going to sleep at a decent hour because of their brain chemistry. Don't be too hard on them because of it - they're probably already feeling the pain of not enough sleep without anyone else pointing it out.