That’s hard to tell without more knowledge about Teachable. It looks like you can’t use their API unless you’re on their Pro tier.
If you know how to get Teachable to control WordPress and can manage groups in Wordpress that get pushed to Discourse, then that could work.
Teachable to Discourse would be best, if it’s possible, but it’s hard to tell if it’s possible. If you have access to their API, then a Discourse plugin that checked their status in Teachable (or better, made teachable the SSO source) could be what you want.