The 16-17 age group is particularly prone to manipulation by their teachers. Unfortunately, they have a bias towards a certain party that uses a lot of red in its logo and whose name begins with the letter L.
I wouldn't blame anything on any left-wing conspiracy by teachers.
However, just given the sheer importance of schooling and family time in their lives, 16-year-olds are more likely to have the opinions of parents and teachers rub off on them. They haven't had enough independent life experience to really develop their own opinions and then, absolutely crucially, to test those opinions against real world experiences. They are effectively challenging their opinions by the proxy of the experience of their parents and teachers, which just has the effect of duplicating views within the country rather than adding fresh ones.
Politically engaged teenagers today have the benefit of much more rolling news and online information, so I don't doubt that teenagers may be getting better informed. But the age at which these opinions are put to the test in the wild is getting later, not younger.