Is it discrimination to passed over for a promotion based on your availability?

I have been passed over twice for a promotion by my employer based on my availability. I am a part-time college student in my Junior year. The only time I cannot work is 12pm-5pm Monday and Wednesday because that is the times that I have my two classes. Technically I am plenty capable of working full-time hours since the store is open 7 days a week 8am-10pm. I've worked at the company for 5 years and know it like the back of my hand, therefore I am plenty capable of being a supervisor, which is the position that I applied for. On my latest interview the manager told me that he needs all his supervisors to have completely open availability before the interview really even got off the ground, he did not even ask for my schedule or resume, which I came readily prepared with. Is this a form of discrimination? Against college students? Ageism perhaps? I don't know, I just feel that it is unjustifiable.