Website Blocked In Single Location

I'm a web developer for an educational institution. We have a site that over 300 students must access each semester. Our site is hosted by a popular shared hosting provider. Our site is accessible to everyone BUT several students living in a particular dorm on our campus. The dorm has a different internet service provider than the rest of the school, so the site IS accessible on the rest of campus.

The dorm ISP insists they have no firewalls blocking any sites including ours. Indeed, the students confirm they have never had a problem accessing any other sites. Our webhost ran some tests and said they do not have a firewall up against the dorm's IP address therefore the problem must be on the dorm's end. Indeed, no one else has problem accessing the site except for these dorm students. Both companies are claiming it must be on the other's end and neither are willing to look into it further.

Some additional tidbits. The site was hosted on a Mac G5 dedicated server in our department in the past and students never complained about access issues. However, that was in our beta phase and less students were accessing the site.

We had one of the students in the building run some ping commands and a tracert. The general ping and tracert commands appeared to indicate the site was working. However, when the student tried various specific ports, the command got stuck on: "connecting to ..." where the dots were our website.

This test seemed to affirm to the server company that the problem was at the ISP level. Again, the ISP insists the problem is not theirs.

My main question is, does anyone know what on earth could be happening and on what end this problem may be on?