What is the difference between "learned" and "learnt"? Is "to learn" a regular or an irregular verb?
No difference, only preference.
"Learned" and "learnt" are both acceptable, American English usually leans towards using "learned" while British English leans towards "learnt".
This can be seen with other words like "burned" and "burnt" which mean the same thing, too. If your teacher has a preference for one over the other, I suggest following it.
According to this website, "to learn" is an irregular verb but you should consult others on this answer.