20 users simultaneously should definitely not cause a problem.
Unfortunately I was unable to check the log at this time for a possible cause.
A common cause are cache functions. I went to check on your site but all there was is a login page to the admin area of a poll system, which would not classify as a website. I guess you probably use an addon domain though, in which case a static page with a link to that, or a redirect to that domain. would be highly recommended.
Thank you!I'm using Moodle which is very suitable for educational sites. Please, can you let me know where I can go to learn how to create "a static page with a link to that, or a redirect to that domain." It was very ackward, to say the least, to be cut off in the middle of a class!