Log on issues

I've been introducing Scratch to three different classes of 5th grade students. For each class, I created one account that I'm having the students share to create and save their files. However, many of the students are getting automatically logged out of the account. Is there a maximum number of people who can be logged in to an account at a time? Is there a way to get around this? Do I need to have each student have their own account? If so, is there an easy way for met to set up and manage that?
Thanks for your assistance!