Mystery of the Roman Ransom in Bloomington, MN

In these two delightful history-mysteries, seven boys in Ancient Rome solve strange crimes . . . thanks to some help from their cranky teacher, a little bit of logic, and a lot of amusing misadventure.
It seems the boys can get nothing right: Their teacher's fiftieth birthday is
long
past (they should have done their math homework); the servant they bought as a present for his birthday isn't just your run-of-the-mill Gaul slave but a courier running for his life; and, to top it all off, the message he's carrying requests the assassination of a Roman senator—the father of one of the boys.