Or, at least in CA, teachers leave because they grow up and need a real job; one that might pay them enough to afford to live in the communities they work in. If you'd like to live in the valley and teach, you better marry well. You can see some of the salaries here [1]; $71k for Belmont means give up any dream of owning a home.
If you have 2 salaries of 71k, you've got 142 which should allow you to buy a home priced at around 600k, more if you've saved for a bit. So that dream is still alive.
[1] http://belmont-ca.patch.com/groups/schools/p/comparing-the-s...