That document implicitly supports my assertions. The school employed duly sworn LEOs to conduct what amounted to a custodial "interview" of this student. Therefore the student retained all 5th Amendment rights. Has a school official such as a principal conducted an interview of the student regarding strictly school policy, no 5th Amendment protections would be needed. Even if the student inadvertently admitted to committing a crime during that non-custodial interview, that admission alone would be inadmissible as heresay in any criminal proceeding, and the student's 5th Amendment rights would immediately apply as soon as a LEO became involved.
Further, no exigency is present in a case such as this to force a student to make incriminating statements when in custody of the school, without notifying the student's parents first and having them present.
It's complicated, but minors have rights and school officials have no standing under in loco parentis doctrine to waive a student's constitutional rights for them. That is reserved specifically to parents and legal guardians outside the school.