Krisette Spangler Carly is right. There is one other book in the series that is considered a supplement or book 2.5. It's called '13 Suspicious Incidents'. It contains 13 individual mysteries involving the inhabitants of Stain'd by the Sea, but it does not advance the series in any way. So, at this time there are 4 books that have been released, but 'Shouldn't You Be in School?' is only the third book in the series.