1) Install this program: http://www.redline-software.com/pub/tkSetup.exe - Run that on your ISA server installing just the "Headers modifier" section (requires the firewall service to restart during install, so 60 seconds loss of internet connection!)
2) Go to Configuration > Add-ins > Web filters and go to the properties of "Headers Modifier" > Parameters > Add
3) Name the rule whatever you want and select "applies to: Processed request headers"
4) Click Add at the bottom, leave action set to "add header", set header name to "X-YouTube-Edu-Filter" and Header value to whatever the set of letters and numbers is assigned to you on your school profile
5) Go to Conditions, set the first condition to be a "policy rule" and have it "Equal" with the value of your outbound web access policy. Now add a second condition with the following settings: "AND" "Web Site Name" "Contains" "youtube" That should do it.
And that should hopefully work Quite a rough set of instructions. Here's a screenshot of my settings to give you a better idea.... (IGNORE THE FIRST SET OF CONDITIONS, THEY'RE WRONG, SEE CORRECTED SECOND IMAGE)