To agree with everyone else - pre reqs are by definition requirements. they vary for different schools, so you don't need to have every pre req finished for every vet school...but you do not to be able to finish the ones for where you are applying, and by the deadline they set for you. here are a few examples of pre-req problems I had.
This was my first year applying, and I did a lot of the research into schools on my own, so I ended up making a few mistakes. I also was generally following the guidelines of pre-reqs for my IS school, which ended up having a lot less than other schools.
I sent in the general app and a supplemental to one school, and then received an e-mail that according to the courses I planned on taking in the spring, I would still be missing a pre req, and that school does not allow any pre reqs to be completed the summer before enrolling, so I had to withdraw my application.
For another school, they only allow one class to be taken over the summer, but I misunderstood and would need to be doing two for them, so I contacted the dean of admissions and he said that it would be fine to take two over the summer, that if i were admitted for Fall 2010 it would not be a problem. So thats an example of some flexibility, but the course does still need to be completed.
I also had one school that contacted me to say the BioChem course I took at UPenn did not meet their requirement, and so I will have to take a higher level course if I would be attending their school in the fall (I applied to 10 or 11 schools total, only one did not accept this class).
Sorry for the really long post, but I hope it helps answer your question