Teachers can have locks. There's nothing stopping themlegally But when it comes down to actully getting a job in a school, it depends on the principal and the school district. Most, and I mean the vast majority of principals, won't hire dread heads. Those who get hired should count their blessings. They are probably the best teachers out there, but being the best teacher doesn't mean you'll get the job, in fact, the best teachers are the ones who don't make it 5 years. They get fed up with the administration.
Literally, 50% of all new teachers don't make it 5 years. Some because they weren't cut out for the job, and a lot because the job doesn't let them actually help kids. You can be loveable all you like, but in the end, the administration stops you from teaching kids the truth.
I grew up around teachers; My mom was one for a few years and quit to become a social worker because she really wanted to help kids. my dad taught for philly for 35 years, my girlfriends dad taught for phily for 35 years, and my girlfriend teaches for philly now. It hasn't change dmuch, except now they teach to the standardized tests. Not only are those tests false and take money away from ailing schools, it stops the teachers for dozens of weeks out of the year from teaching relevant material