I've used JustHost for about 3 years with no problems. I run a class with 30 concurrent users. I had to drop them because they don't support PHP 5.3 yet and I wanted to go with Moodle 2.x.
I switched over to HostGator cheapo shared hosting. That worked great for about five months. Lately I've had problems with requests stalling out at peak times, students not being able to progress through an quiz etc. It doesn't do it all of the time, but enough to be a major annoyance. I've spent a ton of time with Hostgator trying to figure out why the php performance sucks, but no luck. I built a brand new moodle install and I still see performance issues even when trying to give a quiz to 15 students at the same time.
I guess the next thing is VPS, but I'm not sure how much ram is needed.
Is anyone using cheapo shared hosting service for a classroom of students? I'm thinking of checking out siteground for now. it's only $4.00 per month.