There are different levels of kit assembly, from “screw together a couple of pre-drilled components” to “here’s a bag of parts”. Choose a level that comfortable for you.
If you have older and more-or-less responsible students, building can make a great class or after-school project, allowing your students to see “behind the curtain” as to how machines like this function and what goes into creating them.
If none of that sounds exciting, we highly recommend buying a pre-assembled printer.
One option is to buy your first 3D printer pre-assembled and generate enthusiasm for the printing process. When you start having more projects than printer time, look at options like a build-it-yourself $60 Printer or a $200 some-assembly-required kit.