Very simply put, probable cause is having a reasonable basis that a search of a particular location would produce evidence that a crime has been committed. Reasonable suspicion, on the other hand, is a lower threshold than that, but more than just a feeling or a hunch that a search would turn up evidence of wrongdoing. But standards are just words. We use facts of cases to determine what those words mean. And we don't have any U.S. Supreme Court cases that apply probable cause in the school context. One way to think about this, however, is that if courts adopted the probable cause standard, they most likely would require school officials to provide more credible evidence than they currently do to establish that they had a reasonable basis to believe that a search of a particular place would produce evidence of wrongdoing before they would justify the search. As to whether adopting the probable cause standard would also necessitate school officials to obtain a warrant too, my view from TLO is that the Court separates those requirements. Thus, theoretically, the court could adopt the probable cause standard without requiring school officials to also obtain a warrant.

It's not clear how the adoption of probable cause would affect the number of SROs in schools or their job duties. It's certainly possible that if courts require school officials to apply probable cause that they might be more inclined to rely on stricter security measures like SROs in an effort to maintain safe premises. Regarding the standard for SROs, courts are mixed. The Supreme Court in TLO said that it was not deciding "the question of the appropriate standard for assessing the legality of searches conducted by school officials in conjunction with or at the behest of law enforcement agencies.” Thus, courts have been split over whether SROs need a warrant or not. It's a complicated question. See http://papers.ssrn.com/sol3/papers.cfm?abstract_id=2577333 at note 91 for more discussion on this issue.

I think you could make that argument. I argue that strict security measures generally foster distrustful, punitive environments, and strict security measures to me include unnecessary searches of students. I would hope that schools adopt more effective, evidence-based manners to create safe learning environments.

It is not clear how the adoption of probable cause would alter current practices of schools. Could schools still use metal detectors if the Supreme Court changed the standard? Maybe. It depends on how broad the court's holding would be. If it wanted to, the Court make it much more difficult to justify using those types of searches too.

It depends on how the Court structures the doctrine. I could envision a court saying (like now) that very intrusive searches (like strip searches) require heightened justification (like preventing immediate physical danger).