Should I use django admin panel for regular users?
While you can use the admin panel for all users, I don't recommend it.  Security is tight, but not very flexible.  Also, dedicated pages developed for your user functions can be better suited for the job from both the design and functionality standpoints.
Take the time to develop quality pages for your users. You won't regret it.
I am currently trying to make a learning app. The three main users would be the Admin, Teacher and Student. Should I use the django admin panel for the teachers ? It has  a lot of features and is fully customizable and I can choose what a teacher can do or not from there. Is this a correct approach ?