So, you've found that Yahoo Mail is being blocked? That's horrible! As luck would have it, it is possible to work around the internet filters and get on Yahoo Mail anyway.
The best way to unblock Yahoo Mail at school is to use an unblocker. You can get on any site that is blocked with an unblocker. You'll find links to unblockers right at the end of this page.
What is an unblocker?
Unblockers are like a gateway that pass information between the computer you're on and the site you want to go to. The unblocker visits the site you want on your behalf, downloads the page, and forwards it to you. So the site you're trying to visit only sees the unblocker's IP. So actually you are surfing sites indirectly. School firewalls only think you are visiting the unblocker, not the destination site.
A little advice when using an unblocker for Yahoo Mail
It's best to find an unblocker that isn't used much. This is because popular unblockers will be found and blocked by the school. Most unblockers found by doing a search will be filtered sooner because teachers can find them so easily. Even if you find a good one it's a good idea to have an alternative you can use if it gets blocked.
An alternative way to unblock Yahoo Mail at school is by setting up your own unblocker site. This is harder than just using an unblocker but better in the long run because you're in control. Explaining how to do this is beyond the scope of this article, if you search for "How to make your own proxy" you should find help. You can also make some money from this by putting ads on it.
Other ways to get on Yahoo Mail are to use translator sites, for example Google Translate or Babelfish. Also try using Google's cache version of the site. However, you probably won't be able to to login that way, so it's still better to use an unblocker for Yahoo Mail, though these tips are great for many other sites.
More info: Get on Yahoo Mail at school