When you make a worksheet public (i.e., set the sharing level to 'All Redmenta users' or 'Everyone on the Internet'), there should be no problem with anyone being able to complete it at any time. In fact, you can define the worksheets that you want your students to complete as public. In this case, students can find the worksheets more quickly and approach the problems as either a registered profile with a pseudonym or an unregistered user with a pseudonym, and if applicable, reach the solution.