Is plate tectonics a scientific law or theory?
A law and a theory are different things in science and one is not greater than the other.
A Law is an explanation of a specific phenomenon usually of a detailed cause/effect or relationship. Ex.
A Theory is a broader explanation of things that also has predictive powers. In order for ANY idea to become a theory it has to pass numerous tests and "falsifications" (attempts to disprove it), AND its predictions have to be proven. Unless it fails a test, a theory is the best idea we have to explain the events in question, and once it fails a test it will be discarded and we will have to find another idea to replace it. We still and forever will call it the Theory of Gravity because it has passed every test. Same with the Theory of Evolution.
Remember that science follows the evidence wherever it may lead with no preconceptions. It does not begin with the answer and then try to justify it looking only at what agrees with the presupposed answer.
It's a theory. It's too complex to be reduced to a law
A law is almost mathematical in its essence. Newton's Law of motion is a property of motion that almost cannot be derived from something more basic.
Plate tectonicss a property of the behavior of systems, plates, that are far from fundamental, like particles. So any statement about them is a theory.