'This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land.'
Federal laws pursuant to the Constitution have legal status - please cite one prohibiting secession. Cite one requiring states to petition for permission to leave.
The 10th Amendment - a part of the Constitution,states, '[t]he powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.'
There is no delegtion from the states to the federal government to bar secession, nor are the states prohibited from leaving. It looks like it is written into the Supreme law of the land.