Alright, this article is about the UK, not the US. On average anout 40-50% of new teachers change careers after just 3 years in my experience over my last 30 years of teaching.

I do not know UK educational laws, but NCLB (No Child Left Behind) by Bush II, made it difficult for schools to hire unqualified teachers (not impossible, but difficult).

In order to meet yearly Federal evaluations standards all schools must have certified staff in all positions. Failure to do so will negatively impact their evaluation, and therefore, the Federal monies that they recieve as a consequence of the evaluation.

Furthermore, teachers must continue to advance there own education in their accreditation field. This usually means working towards an advanced degree beyond their bachelors. The majority of this happens contemporaneous with them teaching full time, and the teacher is responsible for paying for it, not the school district in most cases. Failure to comply and acquire further education negatively impacts their yearly step grade advancement and their salary scale.