True enough: but that is not something the dean is equipped to cope with on his own. They are pictures of minors and no matter his intentions requesting them (where from?) off his own back sounds like a bad move!

Get in with the police force (in all liklihood they would welcome his connection to the pupils) and provide suppor through that medium; that seems a more logical approach (what he did sounds like a half cocked investigation of his own - which is a bad idea full stop anyway :D).

As it is he will almost certainly be let off with a caution (provided nothing else is found) - but siezing it is all but mandatory. If nothing else to ensure the images are correctly destroyed.