The article's conclusion is absurdly misleasing given the information presented in its source. The US Attorney did not claim that spying on students is legal, nor did he claim that the school staff did not intend to spy on the students, nor did he even claim that no admissible evidence of a crime existed. He claimed the investigators failed to find any such evidence. This is quite possible even if all of the first three conditions are true--it wouldn't exactly be the first time that cops botched protocol and made evidence key to prosecution inadmissible. The statement does not necessarily reflect any opinion as to whether invasive surveillance of students is legal.
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.
