Your route doesn't have a parameter called node, so therefore $node is null.
In order to make node available as a parameter you have to be on a route that has node as a parameter.
So to add the node parameter to your route, you can do like this:
If you don't need to have filters or sorting or other view related things interacting with books or classes then I would set the view up to not display fields and use relationships, etc. Rather I'd configure it to display a view mode of the Student. Then do all display configuration for what should appear about the student in that view mode. I find using ...
thinking the fastest way to do this one is hook_node_insert() ...
if i am following what your saying, you want to set the value of 'field_section' the newly created taxonomy term in tags? if so the following will get you started ....
You can use hook_ENTITY_TYPE_insert which is called after the node has been saved.
This hook runs once the entity has been stored. Note that hook implementations may not alter the stored entity data.