Hi Chris. If it is for a high school and you already know your students then I suggest you don't use self registration at all but instead register them yourself - there are various ways to do this -we use CSV upload. You can do them all in one go. Take a look in the Moodle docs for uploading users. As for them not all having an email address, you can use a fake email if you do it this way.
I have done this (CSV) for years. It works fine, and also allows you to group them in advance, depending on how you set up your CSV file. I have also been able to upload pictures this way, so that students would have a default picture associated with their login. Works well. Good luck.