I've heard people have a lot of problems with prezi's not working before a class starts. Anyone have any similar issues?
For sure, Prezi may not be best suited for every computer. Prezi operates with Flash, so if you have experienced lagging with other Flash media items on your classroom computer, you would have to take this into account when deciding whether or not to use a Prezi in your lesson.
There are also ways to address this issue if you still want to use Prezi. First, you could try to minimize the size of your Prezis by sticking with a more simple layout and using smaller file formats (PNG for images, FLV for videos). In my experience with Prezi, it seems the most intense element for a computer to handle is the zooming effect. For zooming to appear more fluid, you should try to keep each element in the entire presentation within a more limited range of sizes.
I have experienced these issues firsthand, having recently moved from a school with a new English classroom (with a fancy computer and touchscreen) to one with no dedicated English classroom and a bunch of mediocre computers. The change has both affected my Prezi designs and the extent to which I rely on a Prezi to manage the content of a lesson.