Alumn-
Four words starting with alumn- can give trouble. The basic form of this word in its original language - Latin - is alumnus. It originally meant 'a student' or 'pupil'. Although it sometimes means this nowadays, it is increasingly used to mean 'former student' - someone who no longer attends the educational extablishment.
The four forms that start with alumn- are actually one word in Latin, with the ending changed to show different gender and number. (See -a in Latin.) Properly speaking (and academics should always try to speak properly), an alumnus is a male student or former student; an alumna is a female student. The plural for women is alumnae and the plural for men is alumni.
In Latin, if there are both males and females, one should use the masculine form. So a group of former students from a mixed-sex school form its alumni. In English, it is now seen as incorrect and old-fashioned to use a masculine plural to mean 'men and women'. It is better, if clumsier, to say 'alumni and alumnae' ('old boys and old girls', in a school context in traditional British English; 'former men and women students' in Higher Education) - or, for those of us who prefer alphabetical order, 'alumnae and alumni ('old girls and old boys').