[cjg]

No, because if you have the ability to create the hash on the machine then you have the ability to create a hash for a manipulated version.

The blockchain gets round this with consensus.

[Asbostos]

A hacker does yes, but can a kid recreate the hash quickly without the teacher noticing? If some script appear on the net to automate that, Khan academy could react by changing their hash frequently.

[douche]

I think you give the teachers far too much credit.