Django brings a lot of concepts with it. It may be simpler to start teaching with Flask and build up from there until the need for something like Django becomes obvious.

Right. With Flask, I can simply get to the point where I'm rendering templates, styling with CSS, adding assets and populating template variables with data from simple (or complex) functions I've written.

You can take care of a lot with just that.

Eventually, when the application reaches production, you'll have cobbled together a unique subset of Django.

That's the downside of Flask.

And good luck with auditing upstream versions and security in general.

Django projects can be extremely simple.

And most of the "concepts" Django brings with it are extremely important for any non-trivial web application.

Django also makes it harder to mishandle security.

Flask has a very dedicated fanbase.

There seem to be some people who just have a problem with Django choosing sensible defaults for average scale projects.