There are times when all of the things that you would like to use can become very confusing. For instance, you might now know the plural terms of some of the words that you want to use. When you say alumni, this means that you are referring to the plural form of a group of male and female students who have graduated from a school.
When you say alumnus, you are referring to one male student who has graduated from school. There is also a female version of alumnus, also known as an alumna. It is always a privilege for some people to graduate because not everyone would be given the opportunity and the chance to do so.