For the past three weeks, about 40 pupils at Merrifield College have been tirelessly rehearsing their upcoming rendition of Roald Dahl’s Matilda: The Musical, which opens at the Guild Theatre on Thursday. Based on the classic children’s book by Dahl, the production follows the tale of small child Matilda, whose uncontainable genius transforms her school from a dreary hellscape to a joyous safe haven.