Do you have to be a certified instructor to teach cpr to students?

No : But it would best to at least consult with someone who is certified. If these are school age kids it's better that they be taught even if you aren't certified then them having no education about cpr. You should be sure to have the latest recommendations. There have been many recent changes. It's easy and quick to get certified (takes a few hours at best). I would advise getting certified.