§ 7. Position and manner of display
How Current is This? The flag, when carried in a procession with another flag or flags, should be either on the marching right; that is, the flag’s own right, or, if there is a line of other flags, in front of the center of that line.
(a) The flag should not be displayed on a float in a parade except from a staff, or as provided in subsection (i) of this section.
(b) The flag should not be draped over the hood, top, sides, or back of a vehicle or of a railroad train or a boat. When the flag is displayed on a motorcar, the staff shall be fixed firmly to the chassis or clamped to the right fender.
(c) No other flag or pennant should be placed above or, if on the same level, to the right of the flag of the United States of America, except during church services conducted by naval chaplains at sea, when the church pennant may be flown above the flag during church services for the personnel of the Navy.
No person shall display the flag of the United Nations or any other national or international flag equal, above, or in a position of superior prominence or honor to, or in place of, the flag of the United States at any place within the United States or any Territory or possession thereof: Provided, That nothing in this section shall make unlawful the continuance of the practice heretofore followed of displaying the flag of the United Nations in a position of superior prominence or honor, and other national flags in positions of equal prominence or honor, with that of the flag of the United States at the headquarters of the United Nations
United States Code: Title 4,7. Position and manner of display | LII / Legal Information Institute
Two thoughts here, one is the school itself should understand the simple concept that they are a public school in the United States and should be teaching and acting in a manner that reflects this nations heritage above all others. The other is while well meaning and right on moral grounds to remove any nations flag that is displayed equal to the U.S. flag or above it, the right thing to do after that would to have simply left the flag in the schools office so that they may know that any endorsement of this policy is wrong as well. To have simply thrown away the "mexican flag" gives the school the authority to suspend him, even though I agree with his purpose.