Let F be a fibration on a simply-connected base with symplectic fibre (M, ω). Assume that the fibre is nilpotent and T 2k -separable for some integer k or a nilmanifold. Then our main theorem, Theorem 1.8, gives a necessary and sufficient condition for the cohomology class [ω] to extend to a cohomology class of the total space of F . This allows us to describe Thurston's criterion for a symplectic fibration to admit a compatible symplectic form in terms of the classifying map for the underlying fibration. The obstruction due to Lalond and McDuff for a symplectic bundle to be Hamiltonian is also rephrased in the same vein. Furthermore, with the aid of the main theorem, we discuss a global nature of the set of the homotopy equivalence classes of fibrations with symplectic fibre in which the class [ω] is extendable.Another significant result, which motivates us to investigate the extension of a symplectic class, is related to a reduction problem of the structural group of a bundle. To describe the result, we recall a subgroup of Symp(M, ω). A smooth map φ ∈ Symp(M, ω) is called a Hamiltonian symplectomorphism if φ is the time 1-map 2000 Mathematics Subject Classification: 55P62, 57R19, 57T35.