Gosh, I don't know really, but I feel like I have been asked this question before...(HA, just kidding!) There is not much information on this topic on the web, but I did find a reputable doctor, Dr. Vernon Neppe, who has a webpage that is interesting, but very technical. He works in Seattle, WA at the Pacific Neuroscience Institute. His formal definition of deja is "any subjectively inappropriate impression of familiarity of the present situation with an undefined past". PHEW! I haven't found anyone speculating on the neural mechanisms that might control deja vu feelings, but they can be stopped in some people when they take certain drugs that affect your nervous system. Here's Dr. Neepe's webpage, and also a site about "Anomalous Experiences" (What are those?)

Deja vu:
http://www.pni.org/research/anomalous/deja/deja_manifestations.html
http://www.pni.org/books/deja_thumb.jpg

"Anonmalous Experiences"
http://www.pni.org/research/anomalous/

What I did glean from the web was that some people's experiences with deja vu could be connected with a nervous activity disorder such as epilepsy, and can be associated with temporal lobe epilepsy (the temporal lobe is a section of your brain) However, not everyone that has deja vu has epilepsy! (why, I just had the feeling of deja yesterday!) I have included a website, originating from Harvard, that is a forum for people with seizures or other disorders to write to each other. This site might interest you in that it involves real people relating their deja vu feelings and other anomalous experiences.

Mind, Body, and Seisure web forum:
http://neuro-www.mgh.harvard.edu/neurowebforum/EpilepsyArticles/MindBodyandSeizures.html

ALSO, there is another questions/answer site that will have a medical answer to this question posted soon (next few days or so?) so check out "ROOM 103". This is a good source of information on a wide variety of topics, so tell your teacher too!

Room 103: run by young professionals and graduate students from CalTech and UCSB
http://www.room103.com

Happy Hunting!