Kentucky students must be read Miranda rights at school if police are present

Shut up, you are worthless anyway.

It is "legal" because they aren't in custody at the time. Miranda (for adults) requires custody plus interrogation. Hence, no custody, no Miranda. Same reason you can take an adult into custody and not read them Miranda, so long as you aren't interrogating them. (You are, however, required to provide Miranda to juveniles if you take them into custody, pursuant to Ky statute.)