Sub-ticket of MARS-29 Block JTP unassign-to-null in /alumnos dropdown: remove the empty '-' option for the JTP (assignedTeacherUserId) column so a student's JTP can only be reassigned to another teacher, never cleared to none. KEEP '-' for the ayudante column (ayudante is optional). Add server-side guard in reassignTeacher rejecting null when field=assignedTeacherUserId. · mars