There is no doubt that the child cast of School of Rock is the wow factor of the show. Those who play musical instruments do so with stage presence, flair and skill. In particular, Toby Lee's talent as a guitarist goes way beyond his age. The adult cast are actually a little less convincing than the child cast; it feels at times as though they are very consciously acting for a younger audience which can come across as exaggerated. That said, Gary Trainor is a very energetic and fun Dewey.
School of Rock is engaging and fun, and the pace is good throughout. Some of the music is great, although some misses the mark, falling into an awkward place between Lloyd Webber's style of music and rock. Similarly, the storyline (which you might know from watching the film) is compelling most of the time but occasionally drops. If you're willing to take School of Rock with a pinch of salt you will have a lot of fun watching the show, particularly if you enjoyed the film.
Lots of tickets to see School of Rock are quite expensive; however, if you book in advance through London Theatre Direct you can find some seats from only £18. You can catch School of Rock until January 2018.