To try to answer your question
@Citizen of Legoland
They all do the training on the same day because it is related to their particular school and its needs. i.e. curriculum/behavioural needs/safeguarding.
During term time if a teacher is off at short notice they either get a supply teacher in which comes out of the school budget
Or
If it is to cover for an accident on the way to school, or a hold up of some description then for a short period (30 mins to 60) the class is split between other classes.
Or
If the school (Primary only) has a non class based Deputy Head they will cover for the absent teacher
In a High School most of the above applies with other contingencies thrown in.
Safeguarding training, which includes training on radicalism traits to look out for among other safeguarding for vulnerable children, has to take place at least once a year by law and occasionally more.
Safeguarding training also includes EVERY member of staff not just teachers, it includes, cooks, cleaners, welfare staff (formerly known as dinner ladies), site managers as well as School Governors. Logistically it must be seen to be the same training for everyone.
Does that help?
EDIT: ‘occasionally more’ can relate to when, for example, a new form of terrorism is discovered.