I am a (retired, German) teacher, too. And I found it again and again hard to explain this or that to my pupils. If things were worth to be explained I accepted the trouble. Of course, this is also a matter of what pupils they are (age, background).
Ad 1/ In this case the explanation might be that there is a specic file-type for saving charts disconnected from the spreadsheets containing the original data (the .odc files). As charts need to also be available outside of Calc documents - pasted into Writer documents e.g. - there must anyhow be a way to disconnect the from the original source and to order them to keep their data themselves. It might also help to tell the pupils that they need to leave the edit mode of the chart in advance of saving the Calc document. This needn’t be done by “clicking elsewhere”. As with any editing (also of cells e.g.) ‘Quit Edit’ is most simply done by hitting ‘Esc’.
In addition (from my experience) calling ‘Save’ again for an already saved Calc document after some editing of charts will save it all (hopefully as .ods), even if you didn’t leave chart-edit in advance, and not just save the chart. Only if you use ‘Save As…’ (or the document was not saved yet) you get offered the .odc type for saving the disconnected chart.
I would prefer to tell my pupils as a genearl rule that they should give a name to any new document (“poject”?) immediately after creating it and thus to avoid the ‘Untitled’ docs. As “giving a name” is done by saving, our problem should vanish.
Ad2/ As I am not an Excel user, I cannot approach this by comparison. I just would repeat to suggest, not to go MS style and tell “click here”, “click there”. The pupils should be trained to know what they are doing, and that is not clicking, but “editing the chart data ranges” now and “format the second series” then. The clicking should get a distinguishing meaning. Any language should not just consist of “shout” and “shriek” which might be the equivalents of “click left” and “click right”.
From this point of view you may also understand for what reasons I judge it a serious misconception of Calc, not to offer a “named way” (through the menu tree) to enter the edit mode for the relevant components of a chart. Presently there is only the silly > ‘Edit’ > ‘Object’ > ‘Edit’ having selected the chart in advance with the mouse or from the ‘Navigator’.
===Editing 2020-09-14T22:50UTC===
As my attentionI was drawn upon this old topic again by @AlexKemp closing it, I would once more want to tell every teacher reading this:
Dont teach clicking! Teach understanding!
Knowing your pupils or your students you can do this on every level - in different ways, of course, depending on many circumstances.
I would also want to ask everybody having some influence on the further development of LibreOffice, and in specific on improvements to the user interface: Don’t think exclusively in terms of efficiency -assuming a trained professional user- but always be also aware of the fact that you can offer something teaching by being used.
Don’t surrender.
BTW:
-1- Meanwhile charts being copied elsewhere and pasted into a spreadsheet can be reconnected to DataRanges.
-2- Saving charts from inside a spreadsheet to an .odc file is no longer supported (by means I would know of).
-4- The described (as unwanted) way to do it while a chart is edited also is no longer passable.
In an experiment with LibO 7.0.1.2 also an attempt to insert a chart saved to .odc in old times failed.
Is this progress?
Most troubles makes me charts.