Don't know howto do it keeping the same internal records of that students.
But through users.csv file (it's worth a lot registering them trhough csv file for registering the first time and or re-assigning them to work groups later on, etc., from my own expeience in the class)
Instead of numbers, if you get their names and surnames, for instance, you can play a bit with basic Spreadsheet functions (for instance, OpenOffice Calc) to "concatenate" Name and Surname (i.e.: users as NameSurname ).
I did first with Name.Surname (dot in the midle), but with 1.8.x I had problems later on to have Tiki recognize UserPageName.Surname as a WikiWord (no trouble at all with any version if using UserPageNameSurname )
If you need help with concatenate funtion or others at the spreadsheet, ask here (or at edu.tw.o! )
(you'll be wellcome there, and I'd love to read your experience using tiki in educational scenarios)