Group multiple select in form

Hi, 

Let's say I build a form on top of a table with fields

Classroom | Student name | Favorite classmate

I'd like the form to allow each student (indexed by Student name) to pick their favorite classmate from a dropdown list that contains only the students in their classroom (ie. the Student names pertaining to a group defined by the field Classroom). 

I'd like this to be able to scale to a few thousand classrooms, so creating new fields is not an option. I'm an Airtable noob but experienced engineer. 

Thanks in advance!