Note: it's not considered a criminal act, it doesn't mean any of these students (or their families who could also have been photographed, who again could file criminal charges themselves) cannot file civil charges against their school/teachers.
Under criminal law in the US, AFAIK in the absence of criminal intent it's very hard to get charges laid. Courts don't want to extend strict liability onto serious cases, because you'll be landing every doctor in jail for malpractice, negligence or simply not doing enough. I doubt courts would extend strict liability onto any 'spying' infraction, because it would make essentially any wiretapping, etc. to be prosecuted essentially as guilty-until-proven-innocent.
Although I know in the UK sexual offences are strict liability, and as far as I've seen it follows similar lines in the US too, so one inappropriate picture would have likely turned this result 180.
1. School officials take pictures of students at home without their knowledge.
2. Investigation occurs.
3. No charges, because no "criminal intent"
Sure looks like a reasonable conclusion is that the spying is not illegal in the absence of "criminal intent".