You can delete student profiles at your myOsmo page (https://my.playosmo.com).
After you log in, click 'myProfiles' on top of the page, then select the profiles you wish to delete (or click 'Select all' to select all of them) and click 'Delete'.
Please also see our PDF guide to myOsmo here:
https://www.playosmo.com/static/downloads/GettingStartedWithmyOsmoAccountsProfiles.pdf