My partner is completely overreacting to an old classmates death so I am going to tell her so.
How do you think she will feel about you telling her that?
She'll cry a lot and will become depressed for a while
It can be upsetting to see your partner in pain so it is okay to want it to stop but you cannot to want them to get over somebody's death just like that. It will not make things better and it will upset her more. You need to respect how she feels about the classmate's death.
She should get over it and my goal here is to make her stop thinking about this classmate's death.
Everyone grieves in their own ways. You should respect their grief like you would want them to do for you.