checked your calculations: they are right

.
Maybe another way of thinking would be: the job has to be done in a reasonable time, say 5 working days of 6 hours.
That means you need to process 3000/(5*6) = 100 students per hour.
Assuming 5 minutes per student working time you can process 60/5 =12 students.
to get the job done you need 100/12 = 9 assistants.