Retention Reporting

I have been asked to do a retention project for one of our departments, to include length of stay and tracking student movement to other programs, but we don't cohort our students in our system and this is my first time on this sort of project. Does anyone have experience cohorting students in Tableau or do you apply a start term to students on the tables before bringing them into Tableau? (We don't have a data warehouse so I have to export tables into Excel/Access anyway before connecting to Tableau).

Thanks for any tips!

Melinda.