Can I download workbooks and do assignments

That's really vague and ambiguous.

If you have an App that can open the file format the Workbooks are in, you should be able to download the workbooks.

As to assignments, well that depends entirely on what is required to "do" an assignment.

Other than that, you really need to explain in more detail what it is you need to do.

O.k.

I would assume any work in an Excel course would be done in Excel.

If you need Excel, you can download Excel form the App Store.

https://itunes.apple.com/us/app/microsoft-excel/id586683407?mt=8

Other than that, you need to know what else you may be working with, and what the workbook formats are you are going to be using.

Its also very likely, that some things may be different between Excel on a Windows computer, and Excel on an iPad.