Pedagogic pedantry

Off the top of my head, I think I’d make learners aware of the dropping of ne in spoken French :

J’ai pas compris.

On used instead of nous :

Cette année, on passe nos vacances en Italie.

Dislocation of the type :

Nous, cette année, en Italie, on y est pas allés.

Etre used to mean aller :

J’ai été voir mes parents dimanche dernier.

Lack of subject and verb inversion in questions :

Tu as compris?
Tu vas où?

Pronunciation issues as well, il and elle realized as /i/ and /ɛ/ before consonant sounds, ils and elles as /i/ and /ɛ/ or /iz/ and /ɛz/ before vowel sounds. Il y a pronounced more often than not /ja/.

Just a few examples. The idea is to point out the massive differences that we find sometimes between the written norm and actual spoken usage. If I can finish on a personal remark, I must say that, from the little I’ve seen of your posts, I’m not at all worried for your future students.

The only way to avoid such pitfalls is to be part of the French people daily life. Spend time in cafés, listen to people in public transportation, parks, anywhere. You will end up with a French language fine for daily life, and probably not that fine for business, let alone writing a book.

In short, you cannot have it all.

What you write is true for French language as much as English, and it is probably true for any language course around the world for various reasons. One of them is that when a second language is taught to teenagers, the final purpose is not definite and not single. Teachers probably believe they are teaching linguistics, a set of rules so as to be a linguist or a teacher. I have never seen an English language teacher in France caring that much about casual conversation. French people make an especially lousy job at teaching pronunciation, but Japanese teachers of English are the worst I encountered in the world regarding this.

après qu’il soit venu

It makes sense as a rule (indicative mood is legitimate here) but only the very educated will know it. For a lot of French speakers, après qu'il est venu sounds faulty.

malgré que

Very frowned upon by most educated French speakers. I still advise against it (use bien que / même si or malgré + noun). You see it all over the place in earlier authors such as Montaigne or Pascal. This is a good example of arbitrary language normalisation for purely sociological reasons, I think.

J’avais peur qu’elle soit en retard.

French grammar emphasizes on ‘concordance des temps’ whereas usage is more flexible. For instance, ‘subjonctif imparfait’ has dropped out of usage even in written French.

Mon père, il m’a appris à nager.

Dislocation is used all over in speech. I remember our ‘collège’ teachers blaming us for using it in oral speech. It is an idiomatic turn of phrase, though. Maybe not for use in written script but then again, “le nez de Cléopâtre, s’il eût été plus court, toute la face de la terre aurait changé.” (Pascal)

I’d also like to point out that spelling has an awful lot of silly quirks, but with the ‘réforme de l’orthographe‘, it is not as legitimate to blame you for writing cout, ambigüe or elle s'est laissé mourir.

Not a native speaker but still I will give a try. Here is a list of some mistakes I have encountered (and sometimes I have made) during my stint in France. Some of these mistakes came from foreign students of physics, mathematics and the like.

Use of beaucoup DE + noun, i.e. J’ai mangé beaucoup des tomates instead of J’ai mangé beaucoup de tomates.
Je me rappelle de mon enfance instead of Je me rappelle mon enfance. On confond souvent le verbe transitif direct se rappeler avec le verbe transitif indirect se souvenir de.
Je vais visiter mon fils instead of Je vais rendre visite à mon fils. (see http://www.academie-francaise.fr/visiter-son-oncle)
Milles Mercis instead of Mille Mercis, that is, writing down without taking into account that mille is an invariable adjective.
C’est vite instead of C’est rapide.
C’est moi qui a raison instead of C’est moi qui ai raison. see « Moi qui fait » ou « moi qui fais » ?
J’espère que tu es bien instead of J’espère que tu vas bien.
Merci pour m’aider en anglais instead of Merci de m’aider en anglais.
Je n’ai pas des amis instead of Je n’ai pas d’amis.
Je veux vous faire/demander une question instead of Je veux vous poser une question.