I would think that in fact there is more risk in a school setting than as an altar boy.
Yes, and no. An altar boy is basically a helper for the ceremonial parts (lighting and extinguishing candles, bearing the processional cross, etc.). Think of it as a religious stage hand.
As such, they have a fair amount of time alone with the priest as they prepare for religious services or after services. And much of this time is when other parishioners aren't present.
I was a Lutheran acolyte in my youth (their version of an altar boy), and I can see where there would be a lot of opportunities for a lecherous priest. Compounded with that is the fact that the priest is something of a "father figure" and authority figure, so there's a built-in pressure not to report. Also, it was not uncommon for altar boys to have some intent towards entering the priesthood themselves when they got older. Exposing a priest is not a good start to gaining admission to the priesthood.