Davide - consider using pcode which obfuscates the code in fun.m and produces a file called fun.p You could then distribute the p-file to your students who could execute it.
As per the note in the link, the pcode function obfuscates your .m files, it does not encrypt them. While the content in a .p file is difficult to understand, it should not be considered secure. It is not recommended that you P-code files to protect your intellectual property.
