What happens when there are not enough teachers, effective or ineffective to fill classrooms?

Shortage of Teachers in the United States and the United Kingdom
Education can improve people in all walks of life everywhere in the world. Education gives individuals a sense of direction and purpose for life giving most people hope for a brighter future. A country’s development is directly proportional to the level of education its leaders and citizens have. Furthermore, educated individuals are able to understand their fundamental human rights, duties and responsibilities to the society (Lickona, 2009).
Teachers provide valuable and significant roles for society by teaching knowledge and skills to students. They do this when children are at the most impressionable time in their lives. Therefore, teachers can be a major inspiration and encouragement to students’ academic life by accepting and showing compassion to students to make learning enjoyable (Smyth & Wrigley, 2013).
Because teachers are so vital for educating our young people, of course parents and governments want the most effective teachers teaching their young people. What happens when there are not enough teachers, effective or ineffective to fill classrooms?
The United Nations Educational, Scientific and Cultural Organization Institute for Statistics (UIS) estimates for a maximum pupil-to-teacher ratio of 40:1, 24.2 million primary teachers are needed by 2030 and 21 million of those will simply replace teachers that will be leaving the workforce. An additional 3.4 million are needed to for new schools and growth in school-age population. At the secondary level where the ratio is 25 students per teacher there is a need for 44.4 million teachers and 27.6 million of those will be replacing teachers leaving the workforce. There is a need for an additional 16.6 million teachers (United Nations Educational, Scientific, and Cultural Organizations Institute for Statistics, http://tellmaps.com/uis/teachers/#!/tellmap/-1209350075).
Despite, the very vital function that teachers have in the society some of the developed countries like the United States and the United Kingdom still face an acute shortage of teachers to fill all the classrooms in their countries.
HOW LARGE IS THE TEACHER SHORTAGE PROBLEM IN THE US?
HOW LARGE IS THE TEACHER SHORTAGE PROBLEM IN THE UK?