The collection of jobs that have not been done on time

Looking for word for an amount of work that had to be done, but was not and now aggregated into a collection of undone work.

She was falling behind the rest of the class, having 7 unfinished assignments in her ______

You are probably looking for backlog:

a large number of things that you should have done before and must do now:

• I've got a huge backlog of work to do.

(Cambridge Dictionary)

I think backlog is best, but allow me to offer an alternative, in-basket (or in-box).

This refers to a stereotypical office worker having two wire baskets at different corners of the desk, one marked IN and the other marked OUT. The in-basket contained files or messages that had not been taken care of, that were pending work by the employee. The out-basket contained those items that had been completed and just needed to be filed or delivered.

Consider this poor cartoon person's dilemma: