Proof is a fundamental component of mathematics and so, in my opinion, it should be part of mathematical education in schools. It is an important link between topics and so can help pupils achieve a deeper understanding of the “wholeness” of mathematics and, moreover, it is vital for able pupils if they are to be inspired by mathematics and given a firm basis for future mathematical studies.
"Teaching Proof at KS4,"
The Mathematics Enthusiast: Vol. 5
, Article 16.
Available at: http://scholarworks.umt.edu/tme/vol5/iss1/16