Steam allows Portal to be downloaded, but not any other games.

[Ding Dong]

On my school computer, I have limited execute rights and there's a web filter in place.

When I try to download Portal, it'll download fine. If I try to download a non-Valve developed game (such as Civilization 5, Swords and Soldiers HD, World of Goo, etc.) it fails to download. The exact error code is "An error occurred while installing derp (content servers unavailable) I've already changed the download server several times, all of which to a different country. It all results in the same thing, only Portal can be installed. All other games can't be installed. Any ideas what's wrong?

Alternatively, is there some way to download the Mac version of these games on my Windows computer and then transfer it to the Mac?

[Purpuran]

So you're downloading steam games on your schools computers? You probably aren't suppose to be doing that.

Unless your school happens to be using "Steam for Schools", which is limited to only Portal 2

[Ding Dong]

My school is a 1:1 school. In other words, every student receives a Macbook computer for use at school and at home for the entire academic year. In the agreement that I signed with them, games are not expressly forbidden. Nor does it state that my execute rights are abridged. As such, I took it to mean I could install third party applications.

[aiusepsi]

You're getting blocked by the web filter. The difference between Portal and the other games you mentioned is that for the other games Steam uses standard HTTP downloading. So you're a bit stuck.

[Ding Dong]

That seems a tad bit strange considering the fact that Steam handles credit card processing. I would expect Steam to deliver everything via HTTPS.

[aiusepsi]

Different protocols for different things. I imagine one reason that they use HTTP for content downloads is that they want them to be transparently cachable by ISPs and other network operators. Can't do that if the connection is encrypted.

[Ding Dong]

1) I have tons of time during school in which I do absolutely nothing. Reddit is blocked and flash games are only oh so entertaining.

My school considerably more leinient. You can pretty much do anything you want provided you don't damage the computer.