Yes, if it were a Catholic elementary school, the responsibility to send such an email (if there was one) would belong to the principal or pastoral administrator.
At a Catholic university with thousands of employees, you could perhaps ask your supervisor, but I would not take it upon myself to send an email to all my co-workers.
All this is not to say that Holy Days of Obligation are not important. Obviously they are. But – unless you are in a position of spiritual fatherhood to these people – it seems inappropriate to presume to offer a fatherly reminder. If it were me who received such an email, I would think it odd that one of my co-workers at a Catholic university thought I needed reminding or would not be attending Mass at my parish that day.