Dear Ido,
The load on the Galaxy project public server ("main", aka
usegalaxy.org) is often heavy and it's unpredictable when that will happen. As such teaching using main is not, um, recommended.
About 18 months ago, the Galaxy project switched from using main for workshops to using cloud-based Galaxy instances. We use CloudMan to run and manage these servers on Amazon Web Services. This has gone much, much better, and I recommend you look into this option.
Amazon has an education grants program that we are a part of and that I strongly recommend. See
http://aws.amazon.com/grants/. It's not a lot of money, but if they award the grant it will likely be more than you need.
And your question reminds me that I have been meaning to create wiki pages with best practices for running workshops and courses using Galaxy. If I get to that this week, I'll post an early draft page to this thread.
Thanks,
Dave C