Before marking down this review, I must respectfully ask readers to bear in mind that this is a review written by a 22-year-old for a film targetted at children, and as such I may see things a great deal differently from the film's target audience.
This film follows Shaggy, Scooby and Scrappy-Doo as they get jobs as gym coaches at a girl's school that looks like a cross between St. Trinian's and Castle Dracula. All of the students are young girls based on horror stock characters (a vampire, a mummy, a werewolf etc.), and Shaggy and Scooby must help them to beat a group of cadets at a military academy next door.
All of that works well, but the film struggles compared to other entries in the "Scooby-Doo" franchise, because this film is solely targetted at young children. In other films and shows, the rest of the gang provided a serious counterbalance against the slapstick humour provided by Shaggy and Scooby, meaning that older viewers could watch it for the mystery as well as the laughs. In this film, the jokes come one after the other without rest, so older viewers may find the film somewhat tiresome.
The jokes, too, will amuse young children who have not heard them before, but to older viewers, they will come across as being so old that they may have been found in the Valley of the Kings. But then, this film is aimed at kids, and if it keeps them amused, then that's fine by me.
All in all, this is perhaps one best left to the children, and older fans of the series may want to look elsewhere.