Call the school and ask. They're the only ones who know for certain what they expect and need from students entering each class.
Generally speaking, an introductory class requires students to know nothing at all coming in, not even which end the bullets come out of.
A beginning class -- usually -- expects incoming students to know how to load and unload the gun, know where the safety lever is and when to use it, know the safety rules, etc. Good marksmanship is not important at this stage, but you should have some idea how to run the gun. Some (many) schools combine introductory and beginning classes; in these cases, it's good to be basically familiar with how your gun works, but the instructor won't expect it of you.
An intermediate class -- again, usually -- expects that students know basic firearms manipulations including how to clear malfunctions. Students are expected to know how to draw and reholster safely, and have most of their basic equipment issues already worked out. Still no particular speed or marksmanship expectations coming into such a class, but if you've got a nasty trigger jerk or other accuracy issue, you might be better served to start at the beginning class.
An advanced class -- usually -- is built with the expectation that incoming students have all their equipment issues worked out, know how to draw & reholster safely, know how to do speed reloads, and are not going to be intimidated by being introduced to complex skill sets which will require them to safely perform all basic gun manipulations while learning new things at the same time. This means that all basic gun manipulations should be so familiar and so well-drilled that they take little conscious thought. An advanced tactical class also has a certain level of expectations about students' shooting accuracy and (to a lesser extent) speed; if those are still an issue for you, consider taking the intermediate class a second time before joining an advanced-level class.
Again, these expectations will and do vary from one school to the next, and from instructor to instructor within the same school. If in doubt, call and ask. It's the only way to be certain that the class will be a good fit for you.
pax
