In "New Jersey v. T.L.O." the U.S. Supreme Court held that the prohibitions of the Fourth Amendment regarding unreasonable searches and seizures apply to student searches and seizures conducted by public school officials. However, the Court said the legality of a search should depend upon "reasonableness, under the circumstances" rather than probable cause, the standard in a criminal search. Reasonableness depends upon whether the search is justified at its inception and permissible in scope. This paper presents findings of a study that assessed public school principals' knowledge and understanding of the "T.L.O." standard. An instrument comprised of 10 vignettes was mailed to 203 principals in urban schools and to 251 principals in suburban schools. They were asked to indicate whether the search in each situation was justified at its inception and permissible in scope. They were also asked to indicate the sanctions that would be imposed for search-and-seizure policy violations across school settings and sizes. Findings indicate that public school principals possess limited knowledge and understanding of the "T.L.O." standard of reasonable suspicion. Although very little relationship was found across school sizes regarding sanctions they would impose, principals in city schools appeared to favor imposing more severe sanctions than their suburban counterparts, especially with regard to exclusion of students for more than 10 school days. Recommendations for further study are offered. Seven tables are included. Appendices contain a copy of the questionnaire, a list of court cases for the 10 selected vignettes, statistical findings, and copies of letters to the principals. (Contains 91 endnotes and 15 references.) (LMI)