Just to share some insights from our local rural primary school (where I am the head of the parent's association and have regular conversations with the school principal about personnel) and from a primary school in Berlin Kreutzberg (where my sister is a teacher):
Schools get a number of "teaching hours" allotted based on the number of children attending that school (some children might be classed as requiring more hours, i.e. those with certified special learning requirements). It is universally accepted (except by the politicians setting those hours) that these are insufficient to actually provide the supervision and support to have the learning outcomes that are desirable and demanded by government/parents. They also do not cover additional (but foreseeable) requirements due to illness, etc.
No school actually has all of these teaching hours covered because there are simply not enough teachers to go around. Even if, by some miracle, 100% of hours are covered at the beginning of the year, every teacher that leaves during the year due to illness, pregnancy, etc. is pushing down coverage.
Because of the lack of actual teachers, the hours are also increasingly covered by people transferring from other professions. These are counted as full teaching staff, but are obviously not as qualified as actual teachers. Teachers are also expected to do a lot of stuff that is not teaching, i.e. school administration, social work, counseling, etc.
My personal guess is that in most schools, you would actually need about 130% of the teaching hours currently allotted to provide a quality education for all pupils, while schools in special situations (i.e. large migrant populations) would need probably more like 150-200%. In reality, the typical school probably has only 80-90% of allotted hours available (some of which are covered by non-teachers) with schools in special situations often having even less, because they are typically situated in places with more budgetary constraints/not a desirable workplace.