All the subjects are so interrelated that i would be difficult to decide which should be tought first.
Biology deals with Living organism.
Physics deals with properties of matter and energy.
Chemistry deals with chemical composition and properties of a substance.
From the starting that is from the low grade all the subjects should be started simultaneously, so that it will be easy to correlate all the subjects.
Correlating all the subjects helps in understanding and their application.
One more advantage of starting simultaneously would be avoiding confusion.