In the original version of this paper Theorem 1.1 was mistakenly stated for all p. This was also observed by Pikhurko [Pi01] and by Schelp. The theorem is valid only in the linear, quadratic and cubic cases. Namely:
Let k > 2 be a positive integer, and let p=1,2,3. Then t_p(n,K_k)=e_p(T(n,k)), where T(n,k) is the Turán Graph.
Theorem 1.1 is sharp in the sense that for p \geq 4, t_p(n,K_k) is not obtained by the Turán graph. This can already be seen by the fact that the complete bipartite graph G=K_{\lfloor n/2-1 \rfloor, \lceil n/2+1 \rceil} has e_4(G) > e_4(T(n,3)).
A revised version of this paper addressing this comment can be found
in [CaYu04].