Assigning specific lessons
If I assign specific lessons will students be able to do those lessons or will they need to 'unlock' or work their way up to these specific lessons if they're a little further along than they may have already progressed?
You can assign skills that your students haven't reached yet, yes. That's not a problem. It'll unlock those skills for them.
so theoretically it should be possible to assign yourself (as a teacher) different "grammar / verb tense" in the tree which are somewhere in the middle or the bottom?
And then they get unlocked even previous skills are still grey?
Wow...never tried this in a schools teacher/student test account.
The problem I run into in 2017 with my EN->PT tree (when this was possible with a userscript workaround and I didn't need a school classroom to test this) was that previous vocabulary (many words!) was presented to me in the lessons of the LATER "grammar skills".
I had not learned all the words from the previous "vocabulary skills" when I tried to SKIP several grey/locked skills.
And the Duolingo system did not really care and still fired all the previous vocabulary in those fixed wired sentences from the Incubator course at me.
Either the system picks it up for itself....or the course contributors specifically design future lessons (with sentences) like that.
So I quickly gave up after playing around for 1-3 days with Duolingo to SKIP any skills in my tree.
I would imagine that the same still holds true for the French CEFR course which gets designed by contractors.
You NEVER know from the theme-oriented topic names, what content is hidden behind a skill and what implicit grammar or verb tense stuff (imperative, past, future, prepositions, contractions,...) it is throwing at us.
SKIPPING skills and not following the very strict Duolingo skill order is therefore not suggested.
And this was happening to me in my classic EN->PT course where I exactly know what is "hidden" behind the Past/Future/Imperative self-explanatory skill names.
With the latest French CEFR tree (tree14) I would definitely suggest to strictly stick to the given skill order.
Contractors have removed so many "grammar skills" which were previously in the tree2/3 and tree12 and have (fully or partly) spread the content (words + sentences) into other skill names with theme-oriented names.
Why should it make any sense trying to skip a few skills?
Do you maybe know what the school classroom gives a teacher as a different curriculum (e.g for grammar related stuff) outline with better descriptive names and conrete content description so a teacher would want to design his/her own study plan and further customize a Duolingo CEFR tree for the class?
Yes, 'theoretically', that's possible:
Here is the French 'Scope and Sequence' document that tells you which grammar concepts are introduced in which skills: https://docs.google.com/document/d/1a6I35FzOg4Z4a9FUOSX8CcDQoZKPn7SSRZZJCssR8Ts/edit. I don't think being confronted with some vocabulary that was previously introduced poses a big problem. If you're skipping skills, the students are probably too advanced for the earlier ones and already know the vocabulary or won't have a problem learning it themselves.
What your students will see is one purple skill (that they can then click on) with a bunch of gray skills (that they can't click on) all around.
I agree with Thomas.Heiss, though, that if your students are beginners, it's best to not jump too far up the tree. If however, they are advanced and have just started using Duolingo, then that's fine.
If they are advanced, you might also consider encouraging your students to do a checkpoint later than they have unlocked. (Those are the castles.) It will serve the purpose of "testing out" of all previous skills, and therefore unlocking everything before it. This is like a test, and the students will get three "hearts" (in other words, four wrong answers maximum), or they don't succeed in unlocking things.
The castle checkpoints will not count toward Assignment XP, unfortunately.