If a school district adopts Linux and open source then who is going to be the admin in charge of updates, patches, server, network, and desktop maintenance, etc? Competent Linux admins are harder to find than people with at least basic knowledge of Mac and Windows and are likely to cost more too.
Competent admins of any stripe willing to work for typical school district salaries are difficult to find. The only reason many of us do it is because we feel it's the right thing to do. It certainly isn't because we're gonna be rich. How many slashdotters would be willing to give up over 50% of their potential salary in industry because it would help make a school better?
Also, how many teachers know how to use Linux or are willing to invest the time required to learn? After all, they cannot teach their students that which they themselves do not know. These are not insubstantial difficulties.
Again, the same is true for Windows or Macs. I know from years of experience as an admin in a school district that attempting to get the majority of teachers to undertake any sort of computer training for themselves is nigh unto impossible. Free course offered right at work? They complain the time is inconvenient. A course costing money? fugeddaboudit. The OS is simply not relevant. All too many teachers don't want to learn, can't be bothered, and find absolutely no compelling reason to change their attitudes. Without ridding schools of the ridiculous tenure system, there's just about no way to compel them to do what's best for students. As an example, our district still has some that can't successfully push the MSG button on the phone, enter 4 digits when prompted, and retrieve a voicemail message. Do you seriously think those folks will make an effort to learn something about computing?
If it has syntax, it isn't user friendly.