Why isn't learning an obligation of schools?

In Canada, education statutes talk about the right to attend schools, not the right to receive an education. What this implies is that schools do not have a legal obligation to ensure that students benefit from attending. Maybe if it were made a legal obligation that students were to learn in school, teaching as a profession would be seen as one of the high-status professions rather than one that is debatable. The nature of schooling is hands down an essential service however can you imagine the growth that would occur in communities, in students, and in teachers if learning was made a legal obligation? Why is it not a legal obligation, and since it is not, then what is the purpose of schooling? If learning is not an obligation, then what is the point of assessment in schools? Some may argue that putting a mark on students learning is de-grading because of the difficulties of evaluating a mosaic of different learners with one or two types of assessments. 

Any thoughts on this?