Honestly, that is between the teacher and the teaching institution. If you are looking to teach Excel I would recommend talking with the administration of the desired facility you wish to teach at and find out what, if any, requirements would be. They may possibly already have a program in use currently. I don't think there is any type of regulation for teaching Excel.
It always helps to be recognized though. One good way for that is to test with Microsoft (visit their website to find the nearest testing center); for a fee and - of course - a passing grade, you can be Microsoft Certified. It's nothing really "official", but it's recognized by Microsoft. It's a standard, which is all you really need.