As bib mentioned, dorm is an informal word. Collins agrees.
However, a very interesting Ngram that shows the informal word is steadily making headway into published works.
Normally, I would caution against using such informal language in a thesis. However, if dormitory life is the central theme of your thesis, using the lengthier dormitory over and over again might decrease the readability of your work.
I would suggest deliberately introducing the shorter word early on in your work:
The dormitories (or "dorms") are where...
After that, you should be able to use the word dorms elsewhere in the document.
Really, though, you ought to check with your advisor on this.