Enable Custom Field Updates from Students Table

I need to update a Term Student Field for several batches of students. At Academics > Academic Term > Students, I can select specific students, but "Update Custom Field" isn't one of the available actions. What I think I'm going to do is tag each batch of students, then isolate those tagged batches in the Data Slicer and update the custom field from there. But why can't I just update custom fields directly from the Students table?