How many people can use one username and password to login?

There is a student in my class who gave away his/her username and password to other non-students.  How do you prevent double login (block an unauthorized person to join the school classroom) or limit only 1 login?  Of course we notified the parents and changer his/her password.  

I know that I can login from my laptop and destop.  I use one to present lecture and one to monitor students behavior in class.  I know that user can double logins.

Here is similar question: what if 10 other people know 1 username and password, can 10 people simultaneously login and visit classroom unauthorized (for example you have 20 students in class, and now you see 30 total, 10 unknown and unauthorized attendees).