The Risk is that someone could hack your Minecraft server and get Owner rank, and they could probly find you IP adress
Minecraft will hang if you play online because the Minecraft server IP is (probably) banned from the school network.
...
You could. If you want permission, ask the server host or administrator.
There could be two meanings of what you are asking: World: They would need to provide a download link to their download so you can get it from that source, then type %appdata% in your Windows search when you click on the Windows Flag button. Click on Roaming, then click on the folder called .minecraft, go into the subfolder called Saves or something similar to it. Drag the file with the world in it into the Save folder and then launch Minecraft and check if it's there. Server: You can get the IP address to the host's server and play in multi-player mode. This allows you to access to their world that they have made for the server.
There are no cheats to minecraft. Sorry. But you could download and install mods that can help you get what you want.
You could use one of the many websites that you pay for them to host them to host the server, complete with many Bukkit plugins, ServerCraft being one. But if you want to host a server, and start it out from scratch, you could go to the top link in "related links" and download the "Multiplayer Beta Server" at the bottom of the page. It may help to follow a guide as to setting up the server on YouTube.
Make sure all instances of Minecraft Server are closed, also another application could be using the port. Try doing killall java then run the server.
On Minecraftforums
You just host one and advertise your server or you could buy a server which costs around $250-550 per year and advertise that.
the game itself is clean. the only way in my opinion you could get a virus using minecraft is if you download mods or texturpacks that have viruses on them. but i haven't had any issue's. all so if you are in a server some people might be able to hack you if you give them the opportunity.
try a site called planet minecraft, there are lots of good minecraft resources there.