What if teachers were fired for not doing their job well?

In most professions employees receive warnings that lead to getting fired when they do not perform their job to a certain level of standards. Imagine that teachers who fail their students by ignoring IEPs, playing favorites, grading inconsistently from their colleagues or the Common Core Standards, got fired. When students and parents complain enough, in the current system, teachers who are a year away from retirement can be convinced to retire early. This only accounts for a very small portion of poor-performing teachers. What if teachers had to always do their job well in order to keep their job? What if teachers couldn't hide behind tenure with poor-performance? What if teachers worked hard because they had to? What if teachers didn't have unlimited job security? What if tenure didn't exist?