Two points. First, regarding lack of authentication of the text messages, the LA Times story states:
Quote:
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After hours of arguments, Nelson said that just because the text messages on fighting had come from a phone used by Martin -- but whose account is not in Martin’s name -- there was no way to prove the teenager had written them.
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The critical point being that the phone was not in Martin's name and this weakened any inference he wrote the messages. Another judge might have ruled differently but a judge usually has some discretion in evidentiary rulings.
Second, the statute Spats referred to above, 90.404, is Florida's version of Federal Rule of Evidence 404. Evidence of a person's character or propensity is normally inadmissible to show they acted in accordance with that character trait. However, evidence that a "victim" threatened the defendant may be admissible (in some jurisdictions) even if the defendant is unaware of the threats because it is relevant to show the victim's state of mind. If it is a threat not directed to the defendant specifically, it is inadmissible. The school records, etc. appear to be character type evidence which is normally inadmissible. Some jurisdictions allow a criminal defendant more leeway in proving "reverse 404" evidence than the government.
Let me emphasize that Florida law may be different. I've just stated some general principles that may vary by jurisdiction.