Minecraft is a game that's relatively easy to modify.  A number of modifications have already been developed by the user community, some of which might be beneficial to a school-based program.  In addition, editing the the game's standard textures and skins is also a relatively straightforward process.  This page is designed to provide resources and ideas related Mods, Textures, and Skins.

Skins

Skins are the texture graphics that are applied to the 3D player avatars.  They are very simple to edit and surprisingly small image files saved in a .png format.  To change an avatar's skin, you must first log into your account on Minecraft.net, choose Profile, then select and upload a new skin for that account.  That information is then "pushed out" and follows that account regardless of what server (or local machine) is used.

You can use any image editing tool to change or create a custom skin.  GIMP is a free and easy to use solution.  However, it is a bit difficult to tell how the final product will actually render.  So, many players use tools like MCSkinEdit to create (and preview) new skins.  This tool is also very handy for capturing a screenshot of your avatar.  Note:  When first using the program, you'll be prompted to enter the address the skintest2.jar...  Be sure to change the address to:  

http://gf.web44.net/skinedit/skintest2.jar
or
http://dl.dropbox.com/u/15956363/skintest2.jar

The editor looks like this:

In addition, you can also find a number of already created skins at Skindex.com and Minecraft-skins.com (Be sure to preview these, as they may not be appropriate for school.)

Mods

Mods, or modifications, are custom changes to the game that change the game's basic functions.  They are not officially supported by Mojang, but are acceptable to use.  Some may provide features or functionality to the game that might enhance your school program.

Find Mods and more at:  http://www.planetminecraft.com/

Texture Packs

Texture packs are graphic files that replace the games base graphics.  They simply replace the images that are applied to the game's 3D models.  Some texture packs use higher resolution images that can give the game a less pixelated feel if desired.

Find Texture Packs and more at:  http://www.planetminecraft.com/