I used to teach Tableau too and while you're right, the tool still becomes useless after graduation because of the ~$1000/yr price tag.
From there, tableau becomes a "learn it just to get hired at a company that might use it" tactic, which is a valid job hunting strategy but is not something useful to learn in a data viz class, especially since there are other data viz tools the students can learn and keep using after graduation.
Are you teaching how to use Tableau (the product) or Data science/BI? Because I don't think you should be learning how to use a specific software product at school unless it's vocational.
That said, I don't see why any student couldn't continue using the free Public version.
From there, tableau becomes a "learn it just to get hired at a company that might use it" tactic, which is a valid job hunting strategy but is not something useful to learn in a data viz class, especially since there are other data viz tools the students can learn and keep using after graduation.