In general, I was under the assumption that objects created were created instantly, with a limit on mass that we should discuss. Maybe a couple tons or so?
Can we just say that there's some sort of force (perhaps divine) that prevents him from creating illicit substances or prevents such creations from existing long enough to be used?
They do happen instantly, when he's finished speaking. It is possible that the actual speaking could take a while, though. As noted: size/mass, complexity, and distance all increase difficulty. Presumably, speed (if created in motion) does as well, and making multiple objects (especially if dissimilar or not at least conceptually grouped). Longer and more precise phrasing helps to some extent, but there are presumably limits he just can't surpass yet. (I am not trying to hide behind the fact that "it takes a long time" is less arduous a limitation when I'm writing than it would be to actually describe an object you want for five hours straight.)
So let's say if he might be able to create an object weighing a ton or two, it would have to be fairly uniform (like a big rock), pretty much within arm's reach, and not already moving (probably it should have to be on solid ground too). But he could pretty easily yell "Baseball!" and have it be flying at someone pretty damned fast, and most likely a deck of cards is easy enough. A fruit basket is...well, it obviously isn't gamebreaking but it would either take a while or he'd probably screw it up (whoops, all apples or it tastes funny). I'm going to assume that very large numbers of objects are hard even if they're small/simple because shouting a violent sandstorm at someone could get really broken.
There should probably be a hard minimum on how long it takes to make a permanent/real object, even if it's simple and no one would care. I mean, I don't think it should take an hour to make a real true apple, but the power needs to be in check somehow. (It is intentional that it might well be a hell of a lot faster than making an object by hand, but still.) I'm guessing he's supposed to ask someone before doing anything even slightly questionable.
I'm okay with just, say, going with the idea that Tommy's dad is capable of somehow investigating if Tommy's been doing anything he really shouldn't. And Tommy knows this, and probably doesn't know how it works or how to evade it. So nothing a moderately stern father would kick his ass for, which probably covers most serious abuses anyway. (It's possible he is occasionally in touch to make sure that dangerous-sounding objects are things the teachers already know about, i.e. that Tommy is only doing that stuff under supervision.)
Tenjin was, incidentally, known as a god of natural disasters before the scholarship thing eclipsed that in the general view of him. Presumably he would not actually strike his son with lightning, but it's the kind of thing that makes you respect the guy's potential anger.