Hi Fiona,
Another great question. This is a good one because it is a common use case in schools, ie. you want to force the data to be entered and make it easy for users to select from a list that may (or may not) change over time.
I think FormRanger is the one you want to check out (you can also find this in the Resource Library > Useful Add-ons).
This add-on allows you to dynamically populate Form fields from a list in a Sheet – VERY handy!
I would use the same Sheet for keeping track of the list of student names as the one for also capturing the raw Form data.
You will want to make the instructions on the Form explicit for users.
Let me know how you go.
Rob