>>> To come to Bulgaria as intern of AIESEC's exchange program you need to show in the Labour Bureau in the town of the hosting Local commottee an official Note/Document from your university stating that you are student there and that you need this internship as part of your education - like a compulsory practicum. You should check under what circumstances you could get this document from the university because it varies from university to university and from country to country - in some countries you could be provided with such Note/Document if you are undergraduate, or after you enroll for a second bachelor degree or a master degree.
>>> And after you take this document from the University you should bring it to your Ministry of Education (or another ministry - it's up to the local reality - that deals with approvemnt and quality standards tracking of universities in your country) to ask them to put their official seal on your University Note/Document - this seal would mean that your university is a legal university and is accreditted from the State and the Ministry of Education.
You could ask in your university where exactly to go for such a seal, they should be informed about this procedure and the details about it.
So, just keep it in mind when you apply for Bulgaria!
BTW, these rules are also for Germany and France.