More academic is generally going to mean citations. You show a background for what you are asserting. You need to support what you are saying and use formal English, not colloquialisms. This also means contractions are out unless you're quoting someone.
If this is your bachelor's thesis, then isn't it your last paper/project before you graduate? If so, then look at your best work and see what it was like. If it's not, and you're using the term thesis
when you really mean the term essay
, why not ask your professor what s/he means?