The younger the kids, the more chance you have to shape and influence them. At that level, you have a great deal of opportunity to change kids and influence them. At the middle school level, it lessens a little bit, but you've still got that window. At the high school level, it's a tough battle. The train is already moving and you can't do a lot to derail it.
High schools are the most difficult places to institute change.
Some high school teachers see themselves mainly as curriculum specialists-content experts. They focus more on subject matter than on kids. But when you observe high school teachers, you'll notice that the most successful ones talk about kids more than they talk about curriculum. The real successful high school teachers are student magnets. They just draw kids to them.