The typical "school type locker" is "ok" for a temporary storage manner in order to comply with certain 'laws' that say "metal container". That's about it. Realize that there are a few different VERY EASY manners to enter into said lockers, they aren't very "secure" at all. IE just look at how the inner locking mechanisms work. And then look at the base of the door.
They are just one-half step up from "leaning your rifle against the corner of your closet" in the sense of "out of sight-out of mind". How secure is that? Now, that's not to say that they can't be 'fortified' and/or 're-inforced', but lipstick on a pig is still just lipstick on a pig.